record 2013/7: a revised classification system for

Geological Survey of Western Australia

RECORD 2013/7

REVISED CLASSIFICATION SYSTEM FOR REGOLITH IN WESTERN AUSTRALIA, AND THE RECOMMENDED APPROACH TO REGOLITH MAPPING

Government of Western AustraliaDepartment of Mines and Petroleum

Record 2013/7

REVISED CLASSIFICATION SYSTEM FOR REGOLITH IN WESTERN AUSTRALIA, AND THE RECOMMENDED APPROACH TO REGOLITH MAPPING

Perth 2013

Government of Western AustraliaDepartment of Mines and Petroleum

MINISTER FOR MINES AND PETROLEUM

Hon. Norman Moore MLC

DIRECTOR GENERAL, DEPARTMENT OF MINES AND PETROLEUM

Richard Sellers

EXECUTIVE DIRECTOR, GEOLOGICAL SURVEY OF WESTERN AUSTRALIA

Rick Rogerson

REFERENCE

The recommended reference for this publication is:

Geological Survey of Western Australia 2013, A revised classification system for regolith in Western Australia, and the recommended

approach to regolith mapping: Geological Survey of Western Australia, Record 2013/7, 26p.

National Library of Australia Card Number and ISBN 978-1-74168-485-8

Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned

in the text are referenced using Map Grid Australia (MGA) coordinates, Zone 50. All locations are quoted to at least

the nearest 100 m.

Published 2013 by Geological Survey of Western Australia

This Record is published in digital format (PDF) and is available online at <http://www.dmp.wa.gov.au/GSWApublications>.

Further details of geological products and maps produced by the Geological Survey of Western Australia

are available from:

Information Centre

Department of Mines and Petroleum

100 Plain Street

EAST PERTH WESTERN AUSTRALIA 6004

Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444

www.dmp.wa.gov.au/GSWApublications

iii

Contents

Abstract ..................................................................................................................................................................1

Introduction ............................................................................................................................................................1

Classification system ..............................................................................................................................................2

Landform codes (primary codes) ....................................................................................................................3

Landform code qualifiers (landform element or pattern) .........................................................................7

Regolith age .............................................................................................................................................7

Compositional codes (secondary and tertiary codes) ......................................................................................9

Assigning secondary codes ....................................................................................................................10

Duricrust ...............................................................................................................................................................10

Regolith narratives on geological maps ...............................................................................................................12

Map symbology .............................................................................................................................................13

Ordering of regolith units on geological maps ..............................................................................................13

Named lithostratigraphic units ......................................................................................................................13

Regolith mapping workflow .................................................................................................................................13

Conclusion ............................................................................................................................................................18

References ............................................................................................................................................................18

Figures

1. Diagrammatic relationships of regolith units in Western Australia, showing primary regolith codes .........9

2. Selection of secondary codes .....................................................................................................................11

3. Selection of secondary codes for exposed regolith derived from igneous and high-grade

metamorphic rock .......................................................................................................................................12

4. Examples of orthophotographs...................................................................................................................14

5. Landsat images from the Mount Sandiman map sheet ...........................................................................15

6. Examples of Landsat images used to generate regolith–landform coverage .............................................16

7. Recommended workflow for compilation of regolith–landform maps ......................................................17

8. Example of alluvial plain ...........................................................................................................................19

9. Pedogenic calcrete ......................................................................................................................................20

10. Groundwater calcrete, Gascoyne area ........................................................................................................20

11. Three different generations of colluvium ...................................................................................................20

12. Colluvial fan, Glacier National Park, USA

13. In situ ferricrete at the top of a regolith profile, Gascoyne region. ............................................................21

14. Floodplain deposit adjacent to alluvium ....................................................................................................22

15. Cracks developed in gilgai overlying the Carson Volcanics, west Kimberley ...........................................22

16. Lacustrine deposits and relationship to adjacent sheetwash and sandplain, Minigwal. .............................23

17. Deflation lag, Gascoyne area. .....................................................................................................................23

18. Position of saprolith in a typical regolith profile. .......................................................................................24

19. Regolith and regolith profile, Gascoyne, showing relationship of saprolith, saprock and saprolite ..........24

20. Silcrete, Gascoyne area. .............................................................................................................................25

21. Sheetflow deposit, eastern Yilgarn Craton ..................................................................................................25

22. Tiger bush pattern, Gascoyne area .............................................................................................................26

Appendix

Glossary .................................................................................................................................................................19

Tables

1. Definition of regimes in the RED scheme, adapted from Anand et al. (1993) .............................................2

2. Primary regolith codes for GSWA maps ......................................................................................................3

3. Landform (primary) code qualifiers .............................................................................................................4

4. Secondary codes and qualifiers for regolith composition ............................................................................7

5. Tertiary codes and qualifiers for parent rock or cement type .......................................................................8

6. Example of recommended order of regolith units on regolith–landform and geological maps .................13

7. Resolution and availability of datasets commonly used for regolith-landform mapping in GSWA ..........14

8. Landsat ratios commonly used for generation of regolith–landform maps in GSWA ...............................15

GSWA Record 2013/7 Revised classification scheme for regolith in Western Australia

1

Revised classification scheme for regolith in Western Australia,

and the recommended approach to regolith mapping

AbstractThis Record is a revised version of the Geological Survey of Western Australia’s regolith classification scheme, and a discussion of

the recommended approach to regolith mapping in GSWA. The revision has preserved codes from previous versions of this Record,

added new codes, and provided recommendations on suitable code usage. Thus, it can be used for both existing and new maps.

The GSWA scheme classifies regolith using a set of 11 primary codes. Qualifiers can be used with each primary code for a more

precise definition of landform elements. Optional secondary codes and qualifiers can be used for regolith composition, whereas

tertiary codes and qualifiers are used to denote parent rock or cement type. A series of numeric qualifiers previously used to denote the

relative age of regolith are now used to indicate the degree of regolith induration or cementation. The combined primary, secondary,

and tertiary code approach is hierarchical, meaning that this regolith classification scheme can be applied regardless of scale.

In addition to code revision, this revised Record includes a recommended approach to regolith mapping, and a more comprehensive

glossary. It also discusses the importance of regolith classification and mapping in relation to mineral exploration and bedrock

mapping.

KEYWORDS: Cenozoic, deposition, regolith, regolith mapping, residual deposits, stratigraphy

IntroductionEggleton (2001, p. 101) defined regolith as ‘the entire unconsolidated or secondarily re-cemented cover that overlies more coherent bedrock, that has been formed by weathering, erosion, transport and/or deposition of older material’, and ‘includes fractured and weathered basement rocks, saprolite, soil, organic accumulations, volcanic material, glacial deposits, colluvium, alluvium, evaporitic sediments, aeolian deposits, and groundwater.’ This definition emphasizes the diversity of material that makes up regolith, and the wide range of geological environments in which it is found. Thus, any regolith classification scheme must be comprehensive, flexible, and scale-independent. Furthermore, in order to compare regolith maps, there must be a logical approach to describing and coding regolith regardless of terrain, and the choice of code should rely on observation not interpretation.

The importance of regolith in Western Australia can be assessed using several criteria. In terms of extent, outcropping rock accounts for only 563 846 km2, or approximately 22% by area, of the 2 526 418 km2 land area of the State, the remainder being regolith (Marnham and Morris, 2003). In terms of economic importance, regolith-hosted mineralization (principally iron ore, coal, bauxite, gold, mineral sands) accounted for 82% of the State’s mineral production by value in 2010–11, worth more than $63 billion (Department of Mines and Petroleum, unpubl. data).

An understanding of regolith is important for other reasons. In areas of thick and contiguous cover, regolith

composition and distribution is often the only guide to the composition of the underlying bedrock, as regolith results from the interaction of geology, topography, and climate. Thus, understanding the distribution and age of different regolith types can provide important information on landscape evolution and climatic change (e.g. Pillans, 1998, 2005; Krapf, 2011). Regional geochemical surveys used to search for bedrock-hosted mineralization commonly use regolith as a sample medium. The interpretation of the resulting geochemical data relies on an understanding of regolith composition and whether regolith units are genetically related to the underlying bedrock. Exploration for mineralization at depth has highlighted the importance of identifying suitable sample media that are capable of ‘seeing through’ thick, often transported, regolith. Alternative media to regolith have been used (e.g. water, Gray et al., 2009; vegetation, Reid et al., 2009), although they are not always available, meaning that there is a continuing role for regolith as a sample medium (Morris, 2011).

Provision of a regolith layer on a geological map, coupled with information from regional drilling programs, provides important information to the resources industry. This can include identification of in situ vs transported regolith (i.e. whether or not regolith is genetically related to the underlying bedrock), the depth to the regolith–bedrock interface, and identification of dispersion pathways for target and pathfinder elements and their spatial relationship to regolith minerals. This knowledge of the regolith may help in determining whether or not transported regolith completely masks the geochemical signature of underlying mineralization.

2

Various approaches can be used to classify regolith, including material type and properties, depositional characteristics, landform morphology, formational processes (e.g. chemical and physical weathering), and relative or absolute age. Anand et al. (1993) described and classified regolith according to its composition and position in an idealized landscape profile, and differentiated three major regimes: residual, erosional, and depositional (RED scheme; Table 1). Their classification was formulated in the arid interior of Western Australia where an extensive, deeply weathered mantle above bedrock has been subsequently modified by erosion and deposition to form a variety of regolith types. Fundamental to their approach was the recognition of regolith–landform mapping units, which are areas characterized by a particular association of regolith materials, bedrock geology, and landforms. The GSWA classification extends the approach of Anand et al. (1993) by expanding the depositional regime into its constituent categories (e.g. lacustrine, sandplain), and specifies compositional and provenance attributes using optional secondary and tertiary codes.

The GSWA regolith classification scheme relies on the recording of compositional information about regolith and placing it in a landform context. The scheme has been revised several times (Hocking et al., 2001; Hocking et al., 2005; Hocking et al., 2007), and this version of the Record revises existing codes and presents some new ones. It also provides a framework to produce more consistent and uniform regolith layers (especially for 1:100 000-scale geological maps) by establishing standards for regolith interpretation and mapping, while maintaining flexibility to account for differences in geological terrains and climatic zones. Examples are presented of regolith types commonly found in WA, and a recommended regolith mapping workflow is provided that is based on remotely sensed data and ground truthing.

Classification systemThe regolith layer on a geological map should result from the synthesis of an interpreted regolith coverage (derived from remotely sensed data) and ground observations. Remotely sensed data commonly used to generate interpretive regolith maps include aerial

Regime Characteristics Examples

Residual or Relict Reworked, or degraded materials derived from,

and situated on, an ancient weathered land surface

(residual), or representing the remnants of a more

extensive landform (relict)

Sand above granitic plateaus, duricrust-capped mesas,

and siliceous cappings on the edges of breakaways

Erosional Areas of erosion and removal of material to a level

where the mottled zone, clay zone, saprolite, or bedrock

are exposed, concealed beneath thin soil, or concealed

beneath locally derived, associated sediments

Rock outcrops, areas of poorly sorted stony debris

Depositional Widespread sediments that are increasingly reworked

and redistributed relative to the residual or erosional

source. Depositional deposits can be many metres thick

Sheetwash plains, colluvial fans, saline playas,

sandplain, and drainage channels

photographs, orthophotographs, derivative products (e.g. digital elevation models, or DEM), satellite-based and hyperspectral imagery such as Landsat and advanced spaceborne thermal emission and reflection (ASTER) radiometry, and geophysical data such as aeromagnetics and radiometrics. The balance of direct observation and remotely sensed data varies according to data availability and quality, and the allotted time for data interpretation and field checking.

A regolith classification scheme for geological maps should be objective, simple, and logical, but also flexible enough to allow a detailed regolith subdivision when required. It should be capable of being applied regardless of geological terrain, climatic zone, or scale. Any revisions to an existing scheme must offer some compatibility with coding used on published GSWA maps, and complement established formal lithostratigraphic nomenclature applied to named rock units.

Major considerations in terms of coding regolith are the landform setting in which regolith is found, the processes responsible for its formation (physical and chemical), whether the material is in situ or transported, the degree of consolidation, the extent of weathering, and regolith composition. This information is presented using a primary landform code letter (Table 2), which can be subdivided using a series of subscripts (Table 3). An optional secondary code can be used to designate regolith composition (Table 4), and an optional tertiary code indicates either parent rock or cement type (Table 5). Both secondary and tertiary codes can be further subdivided using a set of qualifiers.

The primary, secondary, and tertiary codes summarize a broad range of environments and processes in addition to compositional information. More detail or greater precision is available through subscript qualifiers. For the idealized code:

A1xbycz

each subscript qualifier (x, y, and z) has a fixed position in the code string, which relates only to the preceding primary, secondary, or tertiary code (A, b, and c in the above example). Thus, any regolith code could have a maximum of six letters (three of which are subscripted) plus a numeric qualifier (now indicating the degree of

Table 1. Definition of regimes in the RED scheme, adapted from Anand et al. (1993)


3

Primary

landform

code

Environment and process Notes

R Residual or Relict Remnant material overlying an ancient land surface. Residual material is derived by in situ

weathering of the underlying rock, the regolith; therefore, it shows no evidence of having

undergone significant transport. Relict material includes deposits of uncertain origin, either

transported or residual, or a combination of both. Transported material may represent remnants

of previous landforms

X Exposed Used for rock and weathered rock. Includes bouldery lag

C Colluvial Proximal mass-wasting deposits grading into sheetwash with a significant to perceptible slope

W Sheetwash Distal slope deposits (including sheetflood) where the gradient is minimal, and drainage is not

clearly defined

A Alluvial/fluvial Alluvium in channels and floodplains. Includes deltaic deposits

L Lacustrine Inland lakes, dune and playa terrain, and some coastal lakes. Includes saline and freshwater

playas and claypans, and minor eolian deposits directly associated with the lake system (e.g.

fringing gypsiferous dunes)

E Eolian Dunes, interdune areas, and sandplain resulting from wind action

S Sandplain May be of mixed origin, including residual, sheetwash, and eolian sands

B Coastal (wave-dominated) Beaches, beach ridges, barrier bars and lagoons, and back-beach dunes, coastal cliffs and

other erosional features (e.g. blow-outs)

T Coastal (tide-dominated) Intertidal and supratidal flats and channels, estuaries, and mangrove flats

M Marine Offshore marine deposits such as coralgal reefs, shell banks, and sea-grass banks

Higher level categories

V Valley Lacustrine, alluvial, floodplain, sheetwash, and colluvial units

K Coastal All wave-dominated and tide-dominated units

D Depositional All depositional units

Table 2. Primary regolith codes for GSWA maps

consolidation rather than the relative age, as discussed below). Only the primary code, describing the broad environmental setting or process, is compulsory. The primary, secondary, and tertiary codes, and many qualifiers, are predefined (Tables 2–5). A tertiary code cannot be used without a secondary code. Note that the number following the code A is explained in the section ‘Regolith age’ below.

GSWA produces an increasing number of products in digital form, and digital databases have taken on more importance. As such, databases cannot store certain character attributes (including subscripted codes and italicized characters), and some codes are used in both regolith and lithostratigraphic nomenclature (e.g. ‘A’ as a primary regolith code, and an Archean age qualifier in lithological coding). Therefore, it has been necessary to develop a system for unambiguous database storage of map codes. In Tables 2–5, each code is shown with its equivalent database code. To distinguish a regolith code from a lithostratigraphic code, the former is prefixed by an underscore (‘_’), and non-subscripted codes are prefaced by a hyphen (‘-’). Codes qualified by a subscript are not separated. For example, iron-rich alluvium (Af) has the database code _A-f, whereas an alluvial fan deposit (Af) has the database code _Af. An Archean felsic volcanic rock is coded as Af.

The GSWA Code Builder, a standalone desktop application, has been developed by GSWA (Riganti et al., 2013) to construct bedrock and regolith codes for GSWA’s hardcopy and digital products. The application can be

downloaded from the DMP website at <http://www.dmp.wa.gov.au/datacentre>.

Landform codes (primary codes)

The primary landform code (Table 2) specifies the environment (landform position) and/or process responsible for the formation or deposition of the regolith (e.g. alluvial/fluvial, lacustrine, or eolian). This is determined by identifying the slope, shape, and expression of the landscape, and the material present. Following from this is an assessment of whether the regolith reflects the current (active) climatic regime or is a relict feature of an older regime (Fig. 1). Most but not all relict features have undergone and continue to undergo erosion. Patterns on aerial photographs and satellite images, remotely sensed and geophysical data, and DEMs can assist field observations in mapping these features.

Eleven primary code letters identify the dominant environment or process, ten of which are used for description and coding of the regolith (Table 2). These are R (Residual or Relict), C (Colluvial), W (Sheetwash), A (Alluvial/fluvial), L (Lacustrine), E (Eolian), S (Sandplain), B (Coastal wave-dominated), T (Coastal tide-dominated), and M (Marine). The eleventh code, X (Exposed), is used to designate areas of outcrop. Amalgamations of these codes can be used to create higher level categories such as Valley, Coastal, and Depositional (Table 2).

4

Primary code qualifiers

Primary code Element Map code Database Usage

Residual or Relict Undivided R _R Commonly used at interpretive stage

In situ (residual) Ri _Ri Requires field checking

Duricrust (residual or relict) Rr _Rr Use more specific coding if possible (e.g.

calcrete, Rk); common at interpretive stage

Sand (residual or relict) Rs _Rs Requires field checking

Transported (relict) Rt _Rt Requires field checking

Exposed Exposed bedrock X _X

Badlands Xb _Xb

Escarpment Xe _Xe

Hill (>90 m relief) Xh _Xh

Low hill (30–90 m relief) Xl _Xl

Rise (9–30 m relief) Xr _Xr

Erosional plain (<9 m relief) Xp _Xp

Colluvial Undivided C _C

Colluvial fan Ca _Ca

Cliff-foot slope Cc _Cc Rarely used

Pediment Ce _Ce Rarely used

Footslope Cf _Cf

Rejuvenated pediment Cj _Cj Rarely used

Landslide Cl _Cl Rarely used

Pediplain Cp _Cp

Scarp-foot slope Cs _Cs Rarely used

Talus Ct _Ct

Sheetwash Undivided W _W

Transitional zone between pediment

and transported regolithWe _We Rarely used

Sheetflood fan Wf _Wf Rarely used

Playa, pan Wp _Wp

Scarp-foot slope Ws _Ws Rarely used

Sheetwash plain with tiger bush

patternWt _Wt New code

Alluvial/fluvial Undivided A _A

Alluvial plain Aa _Aa

Stream bed Ab _Ab Use channel (Ac) instead

Stream channel Ac _Ac

Drainage depression/swale Ad _Ad

Delta Ae _Ae Rarely used

Floodplain Af _Af

Gravel bar Ag _Ag Rarely used

Channel bench Ah _Ah Rarely used

Floodplain with numerous claypans Ai _Ai

Stream bank Ak _Ak Rarely used

Levee Al _Al Rarely used

Meander plain Am _Am Rarely used

Backplain An _An Rarely used

Anastomosed plain Ao

Playa, pan Ap _Ap

Stream bar Ar _Ar Rarely used

Sand bar As _As Rarely used

Table 3. Landform (primary) code qualifiers


5



Terrace At _At

Superficial channel Au _Au Rarely used

Fan/Flood-out Av _Av

Swamp Aw _Aw

Oxbow Ax _Ax Rarely used

Ovoid depression in Eolian sandplain Ay _Ay

Lacustrine Undivided L _L

Fringing dunes Ld _Ld

Freshwater lake deposits Lf _Lf

Fringing bedded deposits Lg _Lg

Halophyte-bearing flats Lh _Lh

Freshwater lake, excluding fringing

depositsLl _Ll

Dune and playa terrain Lm _Lm

Playa Lp _Lp

Saline lake Ls _Ls

Swamp deposits around lakes Lw _Lw

Subcropping bedrock in lakes Lx _Lx

Eolian Undivided E _E

Parabolic dunefield Ea _Ea Rarely used

Blow-out Eb _Eb

Dunefield Ed _Ed

Dune Ee _Ee

Deflation basin Ef _Ef

Interdune flat Ei

Mobile dune Em _Em Rarely used

Longitudinal dunefield El _El

Net-like dunefield En _En

Sand and playa terrain Ep _Ep

Eolian sandplain Er _Er

Sandplain overlying alluvial-playa

terrainEs _Es Use Et preferably

Eolian veneer over colluvium and/or

alluviumEt _Et

Lunette or fringing dune Eu _Eu

Interdune pavement Ev _Ev Include gibber plain areas and deflation

lag between dunes

Swampy swale Ew _Ew

Stabilized dune Ez _Ez

Sandplain Undivided S _S Use for deposits of unknown origin

(residual or relict) or mixed origin

Blow-out Sb _Sb Rarely used

Dune Sd _Sd Use Ee

Gravel deflation pavement Sg _Sg Use for deflation lag or gibber plain

Longitudinal dunefield Sl _Sl Rarely used; use El

Net-like dunefield Sn _Sn Rarely used; use En

Sand and playa terrain Sp _Sp

Undulating sandplain Su _Su

Coastal (wave-

dominated)

Undivided B _B

Table 3. continued

6



Beach (foreshore and backshore) Bb _Bb

Cliffs Bc _Bc

Foredune Bd _Bd

Foreshore Bf _Bf

Backshore Bk _Bk

Back-barrier lagoon Bl _Bl

Mobile dunes Bm _Bm

Boulder beach Bo _Bo

Beach ridge plain Br _Br

Storm beach gravels Bs _Bs

Coastal (tide-

dominated)

Undivided T _T

Tidal bar, in channel Tb _Tb

Tidal channel (subtidal base) Tc _Tc

Tidal delta Td _Td

Estuary Te _Te

Tidal flat (intertidal and supratidal) Tf _Tf

Chenier plain Th _Th

Intertidal flat Ti _Ti

Tidal lagoon Tl _Tl

Mangrove flat Tm _Tm

Superficial channel (intertidal) Ts _Ts

Supratidal flat Tu _Tu

Marine Undivided M _M

Coral reef/bioherm Mc _Mc

Reef flat, backreef or rock flat Mf _Mf

Shell bank Ml _Ml

Plain, nearshore Mn _Mn Rarely used

Plain, offshore Mp _Mp Rarely used

Rocky reef Mr _Mr

Shoreface Ms _Ms

Talus slope or footslope Mt _Mt Rarely used

Relict channel Mv _Mv Rarely used

Table 3. continued

The choice of primary code is largely determined by the amount and type of information available, especially for the primary code ‘R’, which is used for both residual and relict material. ‘Residual’ is used where regolith results from in situ weathering, and is therefore genetically related to the underlying bedrock. ‘Relict’ is used where no genetic relationship has been established between bedrock and the overlying regolith (i.e. the regolith has been transported) or the regolith is of uncertain origin (i.e. either transported or residual). Thus, ‘relict’ is commonly used at the interpretive stage of regolith map compilation, where the genetic relationship of an R unit to the underlying bedrock cannot be reliably established. An example is an indurated capping on top of a mesa, which would initially be coded as relict. Subsequent field

checking may indicate the capping results from in situ weathering of the underlying bedrock, and in this case, the unit would be re-coded as residual. However, if the unit represented the eroded remnant of a previously extensive sheet, formed from transported material that has been subsequently cemented (i.e. is the remnant of a previous landform), the term ‘relict’ would be retained. Even when the original process of formation can be determined, the unit should still be classified as either relict or residual, as the identification of the original process is interpretive, and the GSWA regolith classification scheme is based on observation. For example, a dissected portion of an old alluvial terrace should be classified as relict, even though it may contain information indicative of an origin by alluvial processes.


7

Secondary

code

Composition Composition

qualifier

Database

code

c clay -c

black soil or gilgai -cb

chlorite -cc

glauconite -cg

illite -ci

kaolinite -ck

montmorillonite -cm

smectite -cs

d undivided -d

e evaporite -e

anhydrite -ea

gypsum -eg

halite -eh

f ferruginous -f

gossan -fg

hematite -fh

limonite -fl

goethite -fo

g quartzofeldspathic -g

h heavy mineral -h

apatite -ha

garnet -hg

ilmenite -hi

leucoxene -hl

magnetite -hm

monazite -ho

rutile -hr

zircon -hz

k carbonate -k

aragonite -ka

calcite -kc

dolomite -kd

magnesite -km

l heterogeneous -l

m ferromagnesian -m

q quartz -q

r carbonaceous/organic -r

coal -rc

humus -rh

peat -rp

pyritic -ry

t lithic (rock fragments) -t

u ultramafic -u

w weathered -w

x other mineral -x

aluminous/bauxite -xa

mica -xi

manganese -xm

z siliceous -z

opaline -zo

Table 4. Secondary codes and qualifiers for regolith

compositionLandform code qualifiers (landform

element or pattern)

Optional subscripts can be used to achieve greater discrimination or precision for primary codes. The meaning of a subscript is set by its associated primary code (Table 3). The original set of primary code qualifiers from previous versions of this Record has been preserved, although infrequently used codes are designated ‘rarely used’, and should be carefully considered before being applied; they are not commonly found on GSWA maps. Unless a particular landform can be clearly identified, subdivision of the primary code into a landform element should be undertaken with caution. Subdivision of slope deposits (i.e. C and W) in particular should be undertaken with care; instead, subdivision of these units should emphasize composition and degree of consolidation using appropriate codes. By maintaining the original list of landform codes and qualifiers, published GSWA maps can be interpreted using this version of the Record. The addition of comments to the code tables provides some guidance for more correct and appropriate usage when compiling new maps.

The landform code qualifier also means textural information (in particular related to grain size) can be included. This is important where this type of information is specific to a certain landform and process, such as alluvium in channels (Ac) or on floodplains (Af), which can be designated in terms of both their morphology and grain size. Some code subscripts are based on McDonald et al. (1990), whereas others are from Pain et al. (2007).

Regolith age

Western Australia has a diversity of regolith types and regolith landforms, most of which are of presumed Cenozoic age. On early GSWA maps, most unconsolidated regolith was designated as Quaternary, and relict or residual materials (or deposits known or inferred to include a significant thickness of both Quaternary and older Cenozoic material) were grouped as undivided Cenozoic.

There are few indications of actual regolith age (although see Pillans, 1998, 2005) due to the paucity of material suitable for dating, and the limited number of reliable dating techniques for regolith materials. Thus, most regolith ages are relative, and are based on the position of the unit in the landscape, its stratigraphic position, degree of induration, and the extent to which it has been dissected by more recent processes.

Because of the poor constraints on the absolute age of many regolith units, if precise age data are available, they are shown in the map reference, although not included as a code letter in the map polygon label. This allows units to be assigned a specific age where this is known, or given an age range where dating is less precise.

In previous versions of this Record, the relative ages of more than one undated regolith unit were shown by a whole number after the primary landform code letter, with 1 being the youngest. For example, A1, A2, and

8

Tertiary

code

Parent rock or

cement

Qualifier Database

code

a aluminous cement -a

c chemical or

biochemical

sedimentary rock

-c

chert -cc

dolomite -cd

iron formation -ci

limestone -cl

diatomite -ct

g glacial deposit -g

f iron/(ferruginous)

cement

-f

ferricrete -ff

k carbonate

(cement)

-k

groundwater

calcrete

-kg

pedogenic calcrete -kp

l heterogeneous -l

m metamorphic rock -m

gneiss -mn

pelite -mp

psammite -mm

quartzite -mq

schist -ms

granofels/hornfels -mf

amphibolite -ma

o fossiliferous rock -o

p plutonic rock -p

alkali granite -pa

diorite -pd

dolerite -pl

gabbro -pr

granite -pg

monzogranite/

monzonite

-pm

granodiorite -po

syenogranite/

syenite

-ps

tonalite -pt

felsic -pf

mafic -pi

r

duricrust -r

s siliciclastic

sedimentary rock

-s

conglomerate -sc

mudstone,

siltstone, shale

-sm

sandstone, arenite,

wacke

-ss

Tertiary

code

Parent rock or

cement

Qualifier Database

code

u ultramafic rock -u

dunite -ud

komatiite -uk

peridotite -up

pyroxenite -uy

serpentinite/talc

rock

-us

talc carbonate -ut

v volcanic rock -v

andesite -va

basalt -vb

dacite -vd

rhyolite -vr

trachyte -vt

volcaniclastic -vv

felsic -vf

mafic -vi

w weathered rock -w

saprolite -wp

saprock -wr

z silica cement -z

Table 5. Tertiary codes and qualifiers for parent rock or

cement type

Table 5. continued

A3 denote increasingly older alluvium. However, this approach has limitations, in that it was applied locally or at the map sheet level, and therefore had little validity over a wider area. For example, A1 on one map sheet may not be equivalent to A1 on either the adjoining map sheet or the adjacent tectonic unit. In addition, A1 and C1 on one map sheet may not represent the same period of regolith formation.

In this revision of the Record, a revised usage for numeric code qualifiers is introduced. For a single regolith-landform type, 1 indicates unconsolidated material; 2 indicates weakly consolidated, cemented or indurated material; and 3 indicates consolidated, cemented or indurated material. This usage offers some consistency with that of previous versions of the Record, in that in many cases, increasing degrees of consolidation or cementation can be correlated with increasing age, although this may not always be the case (Krapf, 2011).

In mapping regolith units, 1 is usually assigned to the youngest deposit, as it is usually the most widespread and easily recognized. At the interpretation stage or in the early phases of field mapping, it may be difficult to recognize and code older deposits. Assigning values greater than 1 requires field observation, and is almost impossible to justify using remotely sensed data alone.


9

Figure 1. Diagrammatic relationships of regolith units in Western Australia, showing primary regolith codes

In areas where more than one generation of a regolith–landform unit can be identified, a code number should not be used for unassigned units. For example, if three generations of alluvium can be identified (and coded accordingly as A1, A2, A3), the code letter A should be used for alluvium that cannot be confidently assigned to any one of these three generations.

There is no implied correlation between different regolith units with the same numerical qualifier. For example, an alluvial unit labelled A2 may have a different degree of consolidation (and a different age) from a colluvial unit labelled C2.

In order to avoid generation of excessively complex codes, designating the relative degree of consolidation of regolith is restricted to the primary regolith code. For example, Ld1 and Ld2 can be used to indicate variably consolidated dune deposits fringing a lake system. This approach may mean the deposits are of a different age. Alternatively, L1d and L2m indicate that a more consolidated dune and playa terrain is found with less consolidated fringing dune deposits. Two residual or relict carbonate units of different degrees of consolidation are coded R1k and R2k, respectively.

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Compositional codes (secondary

and tertiary codes)

In order to maximize the value of the regolith layer on geological maps (e.g. for planning geochemical sampling campaigns, or indicating the likely extent of underlying bedrock units) it is strongly recommended to assign secondary and if possible tertiary compositional codes to regolith units (Tables 4 and 5). At the interpretive stage of map generation, compositional information for regolith–landform units may be determined using various satellite images and available bedrock geology exposures. During fieldwork it is essential to ground truth these interpretations.

As with landform codes, the secondary and tertiary codes consist of a limited number of categories. A subscript qualifier may be used to give greater discrimination and precision or to extend the available information (Tables 4 and 5). No list of this sort will ever be complete (e.g. the ‘other mineral’ category of Table 4 could contain a multitude of entries). However the aim is to cover most of the compositional variation seen to date in Western Australian regolith.

Examples of the use of secondary and tertiary qualifiers (with the appropriate database codes) are given below:

Map

code

Database

code

Narrative

Acl _Ac-l heterogeneous alluvium in a fluvial

channel

Af c _Af-c clay-rich alluvium on floodplains

Suq _Su-q quartz-rich undulating sandplain

L2gc _L2g-c consolidated clay-rich fringing bedded

playa deposits

Riff _Ri-f-f iron-cemented in situ ferricrete

Rtf i _Rt-f-f iron-cemented transported ferricrete

Cfgi _C-fg-i iron-cemented colluvium with gossan

fragments

Akk _A-k-k carbonate-cemented calcareous

alluvium

Cz _C-z silcrete-rich colluvium

Apc _Ap-c claypan in alluvial system

Bhz _B-hz zircon-rich beach deposit

Leg _L-eg gypsum-rich lacustrine deposit

Amvb _A-m-vb alluvium derived from ferromagnesian

volcanic rock

The qualifier ‘w’ can be used in conjunction with tertiary codes, and as a secondary and tertiary code alone, to indicate weathering. The predominant usage of ‘w’ as a subscript is in areas of erosional regolith.

When regolith coverage is generated as a complement to bedrock mapping, ‘X’ (i.e. Exposed) is replaced by lithological codes. However, for regolith-only maps, areas of exposed rock can be coded using the regolith classification scheme. Examples of codes are given in the next column.

Map

code

Database

code

Narrative

Xl dw _Xl-dw low hill or rise of (undivided) weathered

rock

Xb gpw _Xb-g-pw badlands composed of weathered

granite

Xr dwp _Xr-d-wp rise of (undivided) saprolite

Xgwr _X-g-wr quartzofeldspathic saprock

Assigning secondary codes

Secondary codes are assigned in a hierarchical fashion. The most common approach is shown in Figure 2, although where there is a need to assign a regolith composition to Exposed (X) igneous or high-grade metamorphic rocks, the classification system in Figure 3 should be used. This approach is intended for use in regolith mapping, where a representation of the overall composition of the bedrock is required rather than its specific lithology.

The secondary code ‘d’ (undivided) is used to indicate that the composition of the regolith is unknown, a common situation before field checking has been undertaken. The secondary codes ‘d’ (undivided) and ‘l’ (heterogeneous) are occasionally interchanged, although they are included in the GSWA scheme for specific reasons. The ‘l’ code is used where the regolith has a heterogeneous composition. Thus, Cl is used for colluvium derived from a variety of rock types, such as material found downslope from interbedded chemical and siliciclastic sedimentary rock intruded by dolerite dykes. Similarly, Rl would be used for a residual or relict unit composed of both siliceous and ferruginous duricrust.

DuricrustCalcrete, ferricrete, silcrete, and other duricrust types (see glossary) are of particular interest in regolith mapping, as they can be mineralized (e.g. nickel laterite, uranium-bearing calcrete). As some are residual, they may help with bedrock mapping, and some have been used as sample media in regional geochemistry programs. In previous versions of this Record, only simple regolith coding has been used for these units, imparting relatively little information. For example, calcrete has been coded _R-k, relict ferruginous material of either local or transported origin (‘laterite’) as _R-f, and silcrete as _R- z. Primary code qualifiers can be used to indicate if the material is residual (i, in situ) or relict (t, transported), and secondary and tertiary codes can be used to impart compositional information. In most cases, whether the duricrust is in situ or transported, and information about its composition, only become evident after field inspection.

Modifications to the regolith scheme introduced here allow for more detailed coding of some regolith units. For example, an undifferentiated calcrete is coded as _Rr-k-k (literally, relict carbonate-cemented carbonate), although a valley-fill carbonate (i.e. relict groundwater carbonate) can now be coded as _Rr-kc-kg. Similarly, pedogenic


11

Figure 2. Selection of secondary codes (Y = yes, N = no)

12

carbonate can be coded as _Rr-kc-kp. Relict opaline silcrete is coded as _Rr-zo-z, and silica caprock developed over granite as _Ri-z-pg.

Residual regolith, such as lag (Carver et al., 1987) and pisolitic laterite (Cornelius et al., 2007) are commonly used sample media in geochemical exploration, in that the process of lag or laterite formation can involve the sequestering of ore or pathfinder elements to mineralization. Thus, maps showing the extent of in situ regolith units are useful in planning regional geochemistry programs. At the interpretive stage of regolith map generation, it is often not possible to determine if the unit is in situ (i.e. residual) or transported (i.e. relict), and regolith units should be coded as relict at this stage. In order to provide the most useful type of information to support regional geochemical exploration, separating which units are residual and which units are relict (and modifying the regolith map layer accordingly) should be an essential part of subsequent fieldwork.

Regolith narratives on

geological mapsAn issue not discussed in previous versions of this Record is the preferred narrative for regolith units on GSWA maps. These maps contain a wide variety of different narrative content and style, in part due to the opportunity

Figure 3. Selection of secondary codes for exposed regolith (X) derived from igneous and high-grade

metamorphic rock

presented to provide additional information to that contained in the regolith code. However, there should be some level of consistency in narratives, in order to allow comparison of regolith codes among different map sheets. The following style guide for narratives is introduced here:

For regolith-landform unit only (qualifier, if appropriate, then primary code):

Examples

Unit Narrative

A (_A) Alluvium

Su (_Su) Undulating sandplain

Where composition is indicated by a secondary code (secondary code, and qualifier, if applicable, then primary code):

Examples

Unit Narrative

Aq (_A-q) Quartz-rich alluvium

Suk (_Su-k) Carbonate-rich undulating sandplain

Where a tertiary code is used (composition then primary code then parent material or cement type):


13

Examples

Unit Narrative

Aqpg (A-q-pg) Quartz-rich alluvium derived from granite

Aqk (A-q-k) Quartz-rich alluvium, carbonate-cemented

The above examples are simple code combinations, and it is anticipated that consistency in narratives will be difficult to attain as codes become increasingly complicated.

Map symbology

In some cases, regolith (especially depositional regime units) may preserve information on conditions of sedimentation or neotectonic activity. This encompasses planar (e.g. folding, and different bedding structures like graded or overturned) and linear features (e.g. flute casts, sole marks). On GSWA maps, these features are shown using symbology used for bedrock, although symbols associated with regolith are shown in purple. Where regolith is developed in situ as a residual unit, it is possible that features of the underlying parent rock are preserved in the regolith, including evidence for deformation. Recording and displaying this information is important, especially in areas of extensive regolith cover, where the only information on the composition and geological history of the bedrock is preserved in the regolith. Accordingly, for residual regolith units only, map symbology used for bedrock can be used for regolith, although regolith symbols are displayed as purple.

Ordering of regolith units on

geological maps

The ordering of regolith units on geological maps involves the listing of primary codes, in the order Alluvium, Sheetwash, Colluvium, Lacustrine, Eolian, Sandplain, Coastal (wave-dominated), Coastal (tide-dominated), Marine, and Residual or Relict. Within each primary code division, the simplest code is listed on the left, and the codes become more complex from left to right (Table 6).

The ordering of these codes is alphabetic based on the secondary (and if necessary) tertiary codes. The least consolidated code is listed in the first line, with more consolidated regolith units listed below, and the ordering of units within any one regolith-landform type.

Named lithostratigraphic units

Formally named lithostratigraphic units of Cenozoic age are common near the coast, and are also scattered through the interior of WA. For these, the conventions for coding lithostratigraphic rock units should be followed. The age of most of these units is reasonably constrained. Previously, Quaternary Q and Tertiary T, or less commonly Cainozoic (now Cenozoic) Cz, have been used to specify age. In keeping with current IUGS recommendations <http://www.stratigraphy.org>, Tertiary is typically no longer used. Instead, Quaternary, Neogene, and Paleogene are used where a more precise age than Cenozoic is possible. The series is used after an ‘N’ or ‘G’ prefix to further proscribe the age; thus Q (Quaternary), Np (Pliocene), Nm (Miocene), Go (Oligocene), Ge (Eocene), and Gp (Paleocene). The mnemonics for named units follow the scheme presently used for rock units on GSWA maps and datasets, with a two-letter capitalized code for a group, followed by a single-letter code for a constituent formation (e.g. QKWt-kl on the map face, Q-KWt-kl in the database for the Tamala Limestone of the Kwinana Group), or a two-letter code preceded by an underscore for an ungrouped formation (e.g. Qro-kl on the map face, Q-_ro-kl in the database for the Roe Calcarenite), and a trailing code separated by a hyphen for lithology, as shown above.

Regolith units derived from named rock units are locally distinguished on GSWA maps. In such cases, the code letter used for the formation can be used as the tertiary code, analogous to the ‘parent rock or cement’, to indicate the specific named derivation. For example, colluvium derived from the Pallinup Formation (itself coded as Ge-PLp-sl) could be coded as _C-t-PLp (data code) and Ct-PLp (map code) if composed of fragments or C-d-PLp and Cd-PLp if a more general code was sought. Limonitic wash from the Robe Pisolite (NM-_rb-cip) could be _W-fl-_rb and Wfl-rb. Potential ambiguity is avoided by the description in the map reference, and in the corresponding narrative field of the database.

Regolith mapping workflowGeneration of a regolith map layer involves interpretation of remotely sensed datasets followed by field checking and refinement of interpreted regolith–landform units. Datasets for generating an interpretive regolith map include aerial photographs, orthophotography, satellite-derived remotely sensed data (Landsat Thematic Mapper (TM5), Landsat ETM7, ASTER, and Spot), Google Earth imagery, DEM and shuttle radar topography mission (SRTM) and derivative products, existing 1:250 000 geological map coverage, previously recorded field data, and geophysical data (airborne electromagnetic (AEM), magnetic, gravity, and radiometric data). The resolution and availability of remotely sensed data are summarized in Table 7.

Alluvium A Ac Af c Af cz

A1

A2

Sheetwash W1 W1t

Colluvium C Cf Cgwp Cq

C1l C1q

C2l C2kk C2vb

Lacustrine Ll1

Ll2

Ll3

Table 6. Example of recommended order of regolith units on

regolith–landform and geological maps

14

Orthophotographs provide high-resolution imagery from which regolith polygons can be directly drawn. Enhancements, such as hill shading (Fig. 4) provide an indication of relief. However, in some cases, the lack of colour contrast means that the composition of different regolith units is difficult to determine.

Landsat TM5 data can be enhanced to distinguish different regolith units in terms of composition (Fig. 5). The three most useful combinations for regolith map generation are the AGSO ratio, the Gozzard ratio, and the 754 decorrelation stretch (RGB754_DCS; Table 8). However, the relationship between colour and mineral composition may vary at the 1:250 000 scale for various Landsat TM5 schemes, so field checking is essential. Various Landsat ratios in particular have proven to be useful in distinguishing regolith composition (Fig. 6). The AGSO ratio (Fig. 6a) depicts clay in shades of red, whereas green indicates more iron-rich areas, and blue is more siliceous material. The Gozzard ratio (Fig. 6b) uses bands 5/7 to show clay, hydrothermally altered rocks, and vegetation as red; green (bands 4/7) depict iron and clay;

Dataset Resolution

Aerial photography High but variable

Orthophotography High but variable

Landsat TM5 30 m

Landsat ETM7 15 m

ASTER 15 m

Digital elevation models

(DEM)

Resolution

SRTM 90 m

Altimeter derived DEM Typically 15 cm

Topographic derived DEM Variable

DEM derivatives Variable

Google Earth Variable — continuous

improvement

Airborne gamma spectrometry Resolution varies depending on

line spacing

Table 7. Resolution and availability of datasets commonly

used for regolith–landform mapping in GSWA

Figure 4. Examples of orthophotographs: a) High-resolution colour-enhanced

orthophotograph; b) same image as a) with hill shading to enhance relief


15

Figure 5. Landsat images from the MT SANDIMAN map sheet (1949). Width of image is approximately

50 km: a) Landsat TM5 321 RGB; b) Landsat TM5 741 RGB; c) Landsat TM5 742 RGB; d)

Landsat ETM7 742 RGB

AGSO ratio Red layer, depicting clay comprises principal component 2 of band ratios 4/3 and 5/7

Green layer, depicting iron, is a ratio of bands 5/4

Blue layer, depicting silica, is a composite of bands 1+7

Gozzard ratio Red layer is a ratio of bands 5/7 depicting clays, hydrothermally altered rocks, and vegetation

Green layer is a ratio of bands 4/7 depicting iron and clay

Blue layer is a ratio of bands 4/2 depicting iron and vegetation

PC1 Principal components from bands 1, 2, 3, 4, 5, and 7 displayed as a single image

RGB741 Red layer, band 7, responds to iron-rich minerals

Green layer, band 4, responds to vegetation

Blue layer, band 1, responds to water

RGB754 Used to differentiate rock types

RGB754_DCS As for RGB754, but decorrelation stretch (DCS) gives sharper colour definition

Table 8. Landsat ratios commonly used for generation of regolith–landform maps in GSWA

16

Figure 6. Examples of Landsat images used to

generate regolith–landform coverage.

Width of image is approximately

7.5 km. See Table 8 for description.

a) AGSO ratio; b) Gozzard ratio;

c) decorrelation stretch (i.e. RGB754_

DCS)

and blue (bands 4/2) iron and vegetation. These and other Landsat images (e.g. RGB754_DCS; Fig. 6c) should be interpreted in conjunction with one another and verified by field checking, as there may be some variation in the relationship between Landsat colours and composition among 1:250 000-scale images.

Potassium–thorium–uranium (KTU) data from radiometric coverage is useful for distinguishing relict, sheetwash, and sandplain units. Gozzard (2006) provides a summary of KTU image usage in regolith mapping. In the usual composite image, potassium is shown in red, thorium in green, and uranium in blue. Potassium is largely found in labile minerals such as feldspars and micas. During weathering, these minerals tend to break down, releasing K, which forms clay minerals. Thorium is usually found in resistate minerals and is therefore less likely to be redistributed during weathering, and may be more common in residual or relict units. Uranium is also found in resistate phases such as zircon and monazite, although it can be leached from labile minerals under oxidizing conditions.

A workflow for compilation of a regolith layer (Fig. 7) covers pre- to post-fieldwork. Pre-fieldwork involves selecting which of the available datasets (Table 8) are the most suited for the scale and extent of the regolith layer, and the composition of regolith. Google Earth is an increasingly important application that allows 3D viewing and continuous updating of images.

When the interpretive regolith map is completed, selected sites should be identified for field checking. At each site, a GSWA site information WAROX point should be recorded, and as well as description of the regolith, a photograph should be taken and recorded in WAROX. Particular attention should be paid to sections where regolith profiles are exposed, as these can provide important information on the extent of weathering, and whether relict units are truly relict or residual and may be suitable for collection of samples for dating (Pillans, 2008). Landsat patterns, the extent and type of vegetation, and the degree of incision of drainage can be related to the degree of consolidation or cementation, which can be validated by field checking. In other cases, field checking can be useful in terms of


17

Figure 7. Recommended workflow for compilation of regolith–landform maps

18

constraining regolith composition. In many of these cases, it is not necessary to visit all occurrences of a particular regolith–landform unit, as sufficient information may be learned from one or two examples and extrapolated to other units.

Following field checking, regolith polygons and codes should be revised if necessary, and an exported version of the regolith layer should be draped over Google Earth and viewed in tilted 3D to check for quality.

ConclusionThis version of the GSWA approach to classification of regolith has involved a revision of code usage, suggestions on approaches to classification, a compilation of useful datasets, and an expanded glossary. This third revision reflects the increased importance of regolith in not only understanding the geology of Western Australia, but also how regolith can be used to increase the prospectivity of the State. The regolith classification is not restricted by terrain, geology, climate, or scale. This revision of the scheme reflects the increased level of knowledge about regolith, the wider availability of remotely sensed datasets, and the need to continually assess how regolith is classified and how regolith–landform map layers are compiled.

ReferencesAnand, RR 2005, Weathering history, landscape evolution and

implications for exploration, in Regolith landscape evolution across

Australia, edited by RR Anand and P de Broekert: CRC LEME, Perth,

Western Australia, p. 2–40.

Anand, RR, Churchward, HM, Smith, RE, Smith, K, Gozzard, JR,

Craig, MA and Munday, TJ 1993, Classification and atlas of regolith-

landform mapping units, Exploration perspectives for the Yilgarn

Craton, Australia: CSIRO Division of Exploration and Mining,

Restricted Report 440R (unpublished).

Carlisle, D 1980, Possible variations on the calcrete-gypcrete uranium

model: Open file report, US Department of Energy, GJBX-53 (80), 38p.

Carver, RN, Chenoweth, LM, Mazzucchelli, RH, Oates, CJ and Robbins,

TW 1987, "Lag" — a geochemical sampling medium for arid regions:

Journal of Geochemical Exploration, v. 23, p. 183–199.

Cornelius, M, Robertson, IDM, Cornelius, AJ and Morris, PA 2007,

Laterite geochemical database for the western Yilgarn Craton,

Western Australia: Geological Survey of Western Australia, Record

data package 2007/9.

Eggleton, RA (ed.) 2001, The regolith glossary: surficial geology,

soils and landscapes: Cooperative Research Centre for Landscape

Evolution and Mineral Exploration (CRC LEME), Canberra,

Australian Capital Territory, 144p.

Gozzard, JR 2006, Regolith–landform mapping: a classical approach

using new imagery, in GSWA 2006 extended abstracts: promoting

the prospectivity of Western Australia: Geological Survey of Western

Australia, Record 2006/3, p. 5–7.

Gray, DJ, Noble, RRP and Reid, N 2009, Hydrogeochemical mapping

of northeast Yilgarn groundwater: Geological Survey of Western

Australia, Record 2009/21, 78p.

Hocking, RM, Langford, RL, Thorne, AM, Sanders, AJ, Morris, PA,

Strong, CA and Gozzard, JR 2001, A classification system for

regolith in Western Australia: Geological Survey of Western Australia,

Record 2001/4, 22p.


Strong, CA, Gozzard, JR and Riganti, A 2005, A classification

system for regolith in Western Australia — an update (2nd edition):

Geological Survey of Western Australia, Record 2005/10, 19p.


Strong, CA, Gozzard, JR and Riganti, A 2007, A classification system

for regolith in Western Australia (March 2007 update) (3rd edition):

Geological Survey of Western Australia, Record 2007/8, 19p.

Krapf, CBE 2011, New insights into the regolith of parts of the Gascoyne

region: Geological Survey of Western Australia, Record 2011/22, 54p.

Marnham, JR and Morris, PA 2003, A seamless digital regolith map

of Western Australia: a potential resource for mineral exploration

and environmental management, in Geological Survey of Western

Australia Annual Review 2002–03: Geological Survey of Western

Australia, Perth, Western Australia, p. 27–33.

McDonald, RC, Isbell, RF, Speight, JG, Walker, J and Hopkins, MS

1990, Australian soil and land survey field handbook (2nd edition):

Encarta Press Pty Ltd, Melbourne, Australia, 198p.

Morris, PA 2011, Fine-fraction gold chemistry of regolith from the East

Wongatha area, eastern Yilgarn Craton, in GSWA 2011 extended

abstracts: promoting the prospectivity of Western Australia:

Geological Survey of Western Australia, Record 2011/2, p. 24–26.

Pain, C, Chan, R, Craig, MA, Gibson, D, Kilgour, P and Wilford, J 2007,

RTMAP regolith database field book and user's guide (2nd edition):

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Exploration (CRC LEME), Open File Report 231.

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techniques: Cooperative Research Centre for Landscape Evolution

and Mineral Exploration (CRC LEME), Perth, Western Australia,

30p.

Pillans, B 2005, Geochronology of the Australian regolith, in Regolith

landscape evolution across Australia, edited by RR Anand and P de

Broekert: CRC LEME, Perth, Western Australia, p. 41–61.

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penetration of transported cover at the Titania Gold Prospect, Tanami

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v. 9, p. 267–273.

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Appendix 1

Glossary

No glossary is exhaustive, and only a selection of items is presented here. The following list contains information on regolith-related topics discussed in this Record. References include Anand (2005), Eggleton (2001), Scott and Pain (2008), Carlisle (1980), Tooth (1999), Schwartz (2005), and the following websites:

http://www.arroyorain.com

http://www.college.cengage.com/geology/resources/geologylink/glossary/a.html

http://www.crcleme.org.au

http://www.esglo.com/

http://www.geology.com/geology-dictionary.shtml

http://www.webref.org/geology/geology.htm

Alluvium

Unconsolidated detrital deposit formed in a stream or floodplain; deposited by a stream or running water.

Alluvial fan — fan-shaped deposit formed where a fast-flowing stream flattens, slows, and spreads, typically at the exit of a canyon onto a flatter plain.

Alluvial plain — a level, gently sloping, or slightly undulating land surface produced by extensive deposition of alluvium, typically adjacent to a river that periodically overflows its banks. Characterized by frequently active aggradation by overbank stream flow and erosion by channelled stream flow. It may be situated on a floodplain, a delta, or an alluvial fan (Fig. 8).

Beach

The unconsolidated material at the shoreline that covers a gently sloping zone, typically with a concave profile, extending landward from the low-water line to the place where there is a definite change in material or physiographic form, or the line of permanent vegetation. At the shore of a body of water, formed and washed by waves or tides, usually covered by sand or gravel.

Blow-out

Usually small, open or closed depression excavated by the wind.

Calcrete

Regolith carbonate accumulations, consisting of variably cemented aggregates largely composed of calcium carbonate, although can include dolomite or magnesite.

Figure 8. Example of alluvial plain: a) Orthophotograph;

b) Landsat TM5 RGB741 image

Pedogenic calcrete — calcrete resulting from accumulation of carbonate in the soil moisture zone (Carlisle, 1980) by movement of percolated rainwater and/or soil water, or as a product of biological activity such as a root respiration (Fig. 9).

Groundwater calcrete — calcrete resulting from the accumulation of Ca2+ and HCO3–, transported laterally by groundwater flow (Fig. 10).

Caprock

Duricrust on top of a hill or upper slope, protecting it from erosion; for example, silcrete derived from weathering of dunite.

20

Figure 9. Pedogenic calcrete

Figure 10. Groundwater (valley-fill) calcrete, Gascoyne area:

a) Overview; b) close view. Red book is 21 cm high.

Cemented

Indurated, having a hard, brittle consistency because the constituent particles are held together by cementing substances such as humus, calcium carbonate, or the oxides of silicon, iron, and aluminium. The hardness and brittleness persist even when wet.

Colluvium

Heterogeneous material of variable particle size (soil with or without rock fragments) accumulated on slopes. Transported by gravity, creep, sheetflow, rainwash, mudflows, or solifluction. Compared with alluvium, colluvium lacks bedding, is more variable in grain size, and contains mostly locally derived material (Fig. 11).

Colluvial fan — a fan built by the deposition of colluvium (Fig.12).

Figure 11. Three different generations of colluvium

Figure 12. Colluvial fan, Glacier National Park, USA

<http://www.arroyorain.com>


21

Consolidated

With regard to regolith, has such firmness and coherence that a tool is needed to take a sample from the outcrop or the material remains coherent after extraction from a coring device.

Drainage depression

Level to gently inclined, long, narrow, shallow, open depression with smoothly concave cross-section, rising to moderately inclined side slopes, eroded or aggraded by sheetwash. Surface may be reworked by modern eolian processes.

Dune

A low mound, ridge, bank, or hill of loose, wind-blown granular material (typically sand, in some places, volcanic ash), either bare or covered with vegetation, capable of being moved from place to place by wind while retaining a characteristic shape.

Lunette dune — an elongated, gently curved, low ridge built up by wind on the margin of a playa, typically with a moderate, wave-modified slope towards the playa and a gentle outer slope.

Dune swale — a linear, level-floored open depression excavated by wind, or left relict between ridges built up by wind.

Coastal dune — a sand dune on low-lying land recently abandoned or built up by the sea.

Duricrust

Regolith material indurated by a cement, or the cement only, found at or near the surface, or as a layer in the upper part of the regolith. The various cement types include siliceous (silcrete), ferruginous (ferricrete, lateritic duricrust), aluminous (alcrete), gypseous (gypcrete), manganiferous (manganocrete), calcareous (calcrete), dolomitic (dolocrete), salty (salcrete), or a combination of these.

In situ duricrust — duricrust developed in the upper part of an in situ regolith profile.

Transported duricrust — duricrust representing transported (redeposited) material, which has subsequently been cemented.

Ferricrete

An indurated material formed by the in situ cementation of regolith by iron oxyhydroxides, mainly goethite and/or hematite. The fabric, mineralogy, and composition of the cemented materials may reflect those of the parent (regolith) material.

In situ ferricrete — found in the upper part of in situ regolith profile (also known as ‘lateritic’ duricrust) (Fig. 13).

Figure 13. In situ ferricrete (‘lateritic duricrust’) at the top of

a regolith profile (white box), Gascoyne region

Transported ferricrete — iron-cemented (ferruginous) transported material (e.g. alluvium, colluvium).

Delta

The low, nearly flat, alluvial tract of land at or near the mouth of a river, commonly forming a triangular or fan-shaped plain of considerable area, crossed by many distributaries of the main river. It may extend beyond the general trend of the coast as a result of the accumulation of sediment supplied by the river in such quantities that is not removed by tides, waves, or currents.

Eolian

Transported and deposited by wind.

Estuary

A semi-enclosed coastal body of water that has a free connection with the open sea and within which seawater is measurably diluted with freshwater from land drainage.

Floodout

The area where a river becomes unconfined at its terminus. ‘A site where channelized flow ceases and floodwaters spill across adjacent alluvial surfaces’ (Tooth, 1999).

Floodplain

A land area adjacent to a stream or river that is subject to recurring inundation (Fig. 14).

Fringing lakebeds

Commonly, bedded deposits along a lake or playa margin.

Gilgai

A landform of small basins and knolls or valleys and ridges on a soil surface. It is produced by expansion and

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contraction following wetting and drying of clayey soils that contain smectite (Fig. 15).

Figure 14. Floodplain deposit (Af) adjacent to alluvium (A/A1).

Ac is alluvium in drainage channel <http://www.

google.com>.

Figure 15. Cracks developed in gilgai overlying

the Carson Volcanics, west

Kimberley. Coin (centre of image)

is 20 mm diameter.

Gossan

The weathered expression of rocks that contained substantial sulfide mineralization. Gossans derived from iron-bearing sulfide assemblages typically consist largely of iron oxides and oxyhydroxides and are a form of ironstone. However, gossans formed from the weathering of iron-poor sulfides (e.g. carbonate-hosted Pb–Zn deposits) give iron-poor gossans. Such gossans may be siliceous or have high manganese content. Gossans commonly exhibit a box work fabric derived from that of their sulfide precursors.

Halophyte flats

Mud flats or salt marshes with abundant salt-tolerant (halophytic) plants near a lake or playa.

Indurated

A rock or soil hardened or consolidated by pressure, cementation, or heat.

Lacustrine

Pertaining to, produced by, or formed in a lake. It can also mean a region characterized by a lake (Fig. 16).

Freshwater lake — a freshwater body of considerable size that is surrounded by land.

Saline lake (salt lake) — an inland body of water situated in an arid or semiarid region, having no outlet to the sea, and containing a high concentration of dissolved salts (principally sodium chloride).

Lag

A usually thin deposit of fragments larger than sand size, spread over the land surface. Most commonly represents the coarse material left behind after fine material has been removed by wind or, less commonly, sheetflow.

Deflation lag — Gravel deflation pavement (gibber plain) (Fig. 17).

Pedolith

Upper part of the regoli th profi le , above the pedoplasmation front, that has been subjected to soil-forming processes resulting in the loss of the fabric of the parent material and the development of new fabrics.

Pisolite

A sedimentary rock made up chiefly of pisoliths cemented together.


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Figure 16. Lacustrine deposits and relationship to adjacent sheetwash and sandplain, MINIGWAL (SH51-7).

Figure 17. Deflation lag, Gascoyne area: a) Gibber plain;

b) detail of a), showing subangular to subrounded

quartz-rich metasedimentary rock clasts

Pisolith

A spherical or ellipsoidal body resembling a pea in shape and which is limited to about 2–64 mm in diameter. May have a concentric internal structure, although concentric lamination is not diagnostic. Most pisoliths have an outer cortex or skin (cutan) composed of gibbsite, boehmite, hematite, maghemite, goethite, and anatase with or without quartz.

Playa

A desert basin with no outlet that periodically fills with water to form a temporary lake. A vegetation-free, flat area at the lowest part of an undrained desert basin, underlain by stratified clay, silt, or sand, and commonly by soluble salts; dry most of the time.

Relict regolith

Material that results from transportation and deposition, and is therefore genetically unrelated to the underlying material. It can include deposits of uncertain origin (i.e. either in situ or transported or a combination of both). Such units may represent remnants of an ancient surface.

Residual regolith

Material derived by in situ weathering and showing no evidence of having undergone significant transport.

Sand plain

A level landform pattern with extremely low relief (typically lacking stream channels) aggraded by active wind deposition and rarely active sheetflow.

Saprolite

Weathered bedrock in which the fabric of the parent rock is retained. Compared to saprock, saprolite has more than 20% weatherable minerals altered.

Saprolith

Typically lower part of the regolith that has retained the fabric of the parent rock. It consists of the saprock and saprolite. The definition may include weathered rocks in which only larger structures including bedding, schistosity, veining, or lithological contacts are preserved. The presence of these fabric elements implies that weathering has been essentially isovolumetric, pseudomorphic, and in situ (Figs 18 and 19).

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Figure 18. Position of saprolith in a typical regolith profile

Figure 19. a) Regolith; b) regolith profile, Gascoyne, showing relationship of saprolith, saprock and saprolite

Saprock

Compact, slightly weathered rock with low porosity. Less than 20% of weatherable minerals are altered.

Silcrete

Strongly silicified indurated regolith, typically of low permeability, commonly having a conchoidal fracture with a vitreous lustre; a type of duricrust (Fig. 20).

Sheetflow

An overland flow or downslope movement of water taking the form of a thin, continuous sheet over relatively smooth soil or rock surfaces and not concentrated into channels larger than rills. Unconfined and found in areas of minimal slope (Fig. 21).

Sheetwash

Material transported by sheetflow.


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Figure 20. Silcrete, Gascoyne area

Figure 21. Sheetflow deposit, eastern

Yilgarn Craton. a) Typical

sheetflow material. Leaf

material indicates direction

of water flow; b) and c)

Fine-scale sedimentary

structures in sheetflow

deposits

Shoreline

The intersection of a specified plane of water with the shore or beach. Shoreline is a boundary line (a line has length but no breadth) between water and land (Schwartz, 2005).

Superficial channel

A not-well-defined surface channel, mainly found on sand or sheetwash plain.

Swamp

Almost level, closed or almost closed depression with a seasonal or permanent watertable at or above the surface, commonly aggraded by overbank stream flow and sometimes biological (peat) accumulation.

Terrace

A level, typically narrow, plain bordering a river, lake, or the sea. Some rivers are bordered by terraces at different levels.

Tiger bush pattern

Banded vegetation pattern separated by bare ground, which is oriented roughly parallel to contour lines. Found on gently sloping (sheetflow) plains (Fig. 22).

Tidal channel

A major channel followed by the tidal currents, extending from the offshore into a tidal flat.

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Unconsolidated

Primary property of looseness of the constituents, which allows it to be crumbled or deformed with the fingers.

Figure 22. Tiger bush pattern, Gascoyne area. a) Pattern is not

easily visible at ground level b) banded vegetation

pattern roughly parallel to slope contours

REVISED CLASSIFICATION SYSTEM FOR REGOLITH IN W

ESTERN AUSTRALIA, AND THE RECOM

MENDED APPROACH TO REGOLITH M

APPINGRECORD 2013/7

This Record is published in digital format (PDF) and is available as a free

download from the DMP website at

<http://www.dmp.wa.gov.au/GSWApublications>.

Information Centre

Department of Mines and Petroleum

100 Plain Street

EAST PERTH WESTERN AUSTRALIA 6004

Phone: (08) 9222 3459 Fax: (08) 9222 3444

http://www.dmp.wa.gov.au/GSWApublications

Further details of geological products produced by the

Geological Survey of Western Australia can be obtained by contacting:

record 2013/7: a revised classification system for

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