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A model for northern Gotland high purity limestones based on a Dynamic Strati-

graphic concept Olof Sandström PhD & Mikael Calner, Prof.

Olof Sandström, Graptolit Geoproject AB, Kapellgatan 10, SE-214 21 Malmö, Sweden, olof@graptolitgeo.se

M. Calner, Dept of Geology, Lund University, GeoBiosphere Science Centre, Lund University, Sölvegatan 12, SE-223 62 Lund,

Sweden, mikael.calner@geol.lu.se.

Introduction Carbonate sequence stratigraphy is normally used by the petro-

leum industry to search for quality petroleum deposits in dy-

namic sedimentary environments. Also, a more academic use

of this involves describing the depositional dynamics and his-

tory of ancient and present carbonate environments. Very few

investigations have been done in order to correlate sequence

stratigraphic concepts to carbonates used for industrial purpos-

es. Pawellek & Aigner (2004) investigated a Jurassic deposit

for ultra-white limestone and concluded that the occurrences

followed a cyclostratigraphic pattern.

On Gotland, five major industry areas deal with extraction of

stone and quality demands. These areas are:

Limestone for industrial minerals purpose. This includes

high purity limestone for metal industry, paints, pulp,

paper, plastics and sugar industry. Chemical demands

are high. Other quality factors may be grindability,

lime reactivity, thermal detoriation, hardness, soft-

ness, yellowness, whiteness.

Limestone and marl for cement manufacturing. Here de-

mands are for a mixture of limestone and impurities

like silica, clay etc. to optimize the sintering process.

Limestone for aggregates and construction. Here quality

demands are for durability, hardness and size. Chemi-

cal qualities are not very important, though this is

sometimes linked to physical parameters.

Limestone for agriculture, forestry and lake de-

acidification. Most important are the CaO-value. The

value should normally not be lower than 50%. Other

demands are on pollutants like lead, manganese and

sulphur.

Ornamental stone for buildings, pavements and grave-

stones. Demands are most often related to the hard-

ness, fissure density, brittleness and durability. Colour

and leaching are two other quality factors.

This paper is a part of the project ―Energy effective production

of lime products‖ (see Sandström 2011a, b for further refer-

ences) and will focus on an example with high-purity lime-

stone for production of metallurgic pebble lime, used as a flux

in the steel manufacturing process. The example is from a site

on northern Gotland and will follow the method suggested by

Aigner et al (1999). The purpose is to show how one may uti-

lize stratigraphic knowledge and basin dynamics to predict

where to find limestone of a specific quality.

Dynamic stratigraphy The concepts of ‗dynamic stratigraphy‘ (Aigner et al 1999;

Pawellek & Aigner 2004), involves a systematic analysis of

sedimentary rock sequences along a hierarchy of spatial and

temporal scales, moving from small to larger levels (Fig. 1):

Microfacies analysis (sensu Flügel 2004) identifies gener-

ally small-scale primary and secondary textures of

limestone as controlled by carbonate-producing biota,

Fig. 1 Silurian palaeogeography and location of Gotland. A Paleogeography of Scandinavia and the East Baltic showing Gotland within the square (modified from Baarli et al. 2003). B Stratigraphic units of Gotland and references). C. Facies distribution of Gotland (modified from Sandström 1998). The square in the

northern part marks the area from where the example is taken in the text.

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water energy during deposition, and fluid dynamics

during diagenesis. At the same time, microfacies types

also mirror the chemical composition and thus the

economic potential of the limestone. For instance, low

-energy mud- and wackestone tend to contain more

insoluble matter and are thus usually less pure than

the high-energy pack- and grainstone.

Petrophysical and facies analysis focuses on larger-scale

groups of microfacies types (MFT cf Flügel 2004)

reflecting similar depositional processes. These

groups of microfacies often show comparable physical

and chemical parameters such as their gamma-ray

signal, whiteness and carbonate content. Thus petro-

physical facies types help to characterise the quality of

economic minerals.

Architectural analysis reconstructs the three-dimensional

geometry of rock bodies as a reflection of the dynamic

evolution of depositional environments. These geome-

tries also allow the assessment of the volumes of pro-

duceable mineral resources.

Sequence analysis provides a genetic framework for the

occurrence of rock types within sedimentary sequenc-

es and cycles. The cycles record changing environ-

mental conditions (e.g. baselevel dynamics). At the

same time, these cycles highlight preferred strati-

graphic levels where economic minerals (e.g. ultra-

pure limestones) may occur.

Stacking analysis deciphers the way shorter-term strati-

graphic cycles change and follow longer-term trends,

controlled by various mechanisms (tectonics, eustasy,

etc.) During exploration, stacking patterns help to

predict the regional occurrence of mineral resources.

Basin-analysis identifies the general patterns of basin

paleogeography, dynamics and evolution. It is thus

possible to deduce general rules for predicting the

distribution of certain facies types that have an eco-

nomic potential on a basin-wide scale.

This hierarchical approach provides a logical basis for under-

standing the small- to large-scale occurrence of industrial lime-

stone.

In this paper we present a generalised dynamic model for the

Gotland limestone and marls focusing on its usage for pro-

specting industrial carbonates of specific qualities. Focus is on

high quality pure limestone, but other usages are also men-

tioned and described.

Fig. 2. A sequence strati-

graphic model based on the reef development and eustat-

ic factors.

Fig. 3. Geologic profile, microfacies, chemistry,

depositional facies, thermal detoriation, cyclic pat-tern and depth analysis of DH 1. The letters a – j

refers to example photographs of different facies in

Fig 4. Thickness of the right hand side of the litho-logical column refers to the system by Dunham

(1962) and the letters at the bottom of the column

are: m= mudstone; w=wackestone; p=packstone; g=grainstone; b=boundstone.

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Geology and Sequence-stratigraphy of Got-

land The Silurian of Gotland consists of more than 750 meters of

limestone, marls and some lesser extent siliclastic siltstones

and sandstones (Fig. 1), reflecting a series of stacked carbonate

platform generations that formed in the intra- to pericratonic

Baltic basin at low latitudes south of the equator (see Calner et

al. 2004 for a recent review). This basin was situated on the

southern margin of the Baltic Shield and the East European

platform (Fig. 1). After extension, followed by tectonic quies-

cence in the earliest Palaeozoic, the south-western margin of

the Baltic Shield was active from the latest Ordovician when

the Avalonia Composite Terrane was amalgamated to Baltica.

A substantial change in basin tectonics that may have affected

the Gotland area is noted in the Silurian, particularly during

Ludlow time. During the Silurian, the Baltic basin was fringed

by a carbonate platform system whereas shales with graptolites

formed in the central and deeper parts of the basin. The lack of

major tectonic structures and the exceptionally good preserva-

tion of the rocks enable good control on temporal and spatial

facies change, i.e., on platform architecture. To the south, the

Rheic Ocean separated the Baltic Shield form the main Gond-

wana continent. This palaeogeographic setting makes the Baltic

basin an excellent target for studies of the relationship between

industrial carbonate prospecting and aspects of carbonate plat-

form evolution.

Earlier works on Gotland sequence- and dynamic stratigraphy

includes works by Erikssson & Calner (2008) and Sandström

(2000). Eriksson & Calner (2008) investigated a time interval

within the Late Ludfordian (Late Silurian) of Gotland, and inte-

grated sequence stratigraphy, carbon isotope stratigraphy, and

Fig. 4. Photographs showing the different facies of DH 1. The position of each picture is indicated by its corresponding letter in Fig. 3. a. Crinoid limestone (grainstone) with abundant stylolites. b. Fragmented limestone (packstone). c. Stromatoporoid limestone (boundstone) intercalated with fragmented limestone

(packstone). d. Reefal fragmented limestone (packstone). e. Stromatoporoid limestone (boundstone). f. Crinoid limestone, partly with micritic matrix (packstone/

grainstone). g. Fragmented limestone with solitary rugose corals and coated grains (wackestone/packstone). h. Reef limestone (boundstone), partly rich in stro-matoporoids. i. Crinoid limestone with abundant stylolites and fine clay intercalations (grainstone). j. Fragmented marly limestone, with abundant fossil frag-

ments and in part rich in crinoids (storm depositions; packstone).

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platform-scale palaeoecological changes. Three depositional

sequences (sequences Nos. 1–3), including two separate peri-

ods of forced regression (falling stage systems tracts, FSSTs)

are identified from outcrop and drillcore studies. The sequence

stratigraphical framework is interpreted to reflect glacio-

eustatic sea-level changes. Based on their facies analysis and

sequence stratigraphical interpretation, two main mechanisms

are suggested as responsible for the Late Ludfordian CIE: (1) a

change in the riverine C-weathering flux towards the 13C end

member following glacio-eustatically induced subaerial expo-

sure of carbonate platforms throughout the tropics, and, (2)

increased photosynthetic activity by benthic cyanobacteria ex-

aggerating the d13C values of precipitated carbonates.

Sandström (2000) suggested a sequence stratigraphic model

based on the development of different reef types and morphol-

ogies (mounds, biostromes, bioherms), presenting an ―ideal‖

development of reef formation is: Axelsro bioherms (small

bioherms) – Hoburgen bioherms (large bioherms) – Kuppen

and Holmhällar type biostromes (biostromes; Fig. 2), reflecting

a shallowing upward depositional system of development

which does not necessarily reflect regression in terms of a sea-

level fall. Instead this system is transgressive, reflecting initial-

ly a rapid rise in sea-level and as the transgressive rate decreas-

es, it moves laterally, filling the new space (i.e. highstand shed-

ding). When the transgressive rate is approximately zero, an

extensive shallow shelf has formed (due to progradation and

flooding), that is ideal for biostrome development (cf. Fig. 2).

Such a sequence is found in the Visby – Högklint – Tofta tran-

sition (Riding. & Watts 1991, Watts & Riding 2000). This

model reflects an ideal transgressive and highstand situation,

and can be used to discuss the dynamics of other intervals.

Example from Northern Gotland. Figure 3 shows a drill-core section from the northernmost part

of Gotland. The area of interest is marked in Fig. 1C. Examples

of different microfacies from the section are shown in Fig. 4.

The area of northernmost Gotland (main island), is dominated

by limestone in the northern part and by marls in the south. The

northern part is exploited by several mining companies for its

high purity limestone, suitable for a range of products and ap-

plications.

Microfacies study of the section and 120 other drillcores

from the area, distinguishes 6 major depositional environments,

each reflecting a MFT: outer ramp marls and marly limestone

(wackestone, packstone), shoals and mid ramp limestone

(fragmented limestone, packstone, grainstone), Reef flank de-

bris and crinoid limestone ((packstone), grainstone), Reef lime-

stone (boundstone, packstone), Stromatoporoid biostromes

(boundstone) and back reef limestone (mudstone, wackestone,

packstone). Putting these into quality types gives that the high

quality limestone are to be found in Reef flank and reef facies

containing mainly grainstone and boundstone with subordinate

packstone (cf Fig. 4).

An architectural analysis yields that there is a standard facies

succession that follows the dynamic cyclic pattern of eustatic

change and thus can be predicted. A 3D-model and fence dia-

gram of a part of the area shows a ‗typical‘ succession from

Fig. 5. Fence diagram showing the architecture and three dimensional geometry of the MFT (Micro Facies Types) from the western part of the exemplified area. The succession is here from shoals and mid-ramp facies (Fragmented limestone) to reef facies (Reef limestone and crinoid limestone) and stromatoporid bio-

stromes (stromatoporoid limestone). This reflects a transgressive phase with its maximum flooding at the mid-part of the reef and reef flank facies. The High-

stand tracts are represented by the uppermost part of crinoid limestone and stromatoporoid limestone (cf Fig. 2).

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outer ramp to reef (Fig. 5). Further north this continues with a

mid-ramp to back-reef succession (Fig 4.). The total ‗cycle‘ of

the limestone part is between 15m and 60m. From the architec-

tural study, volumes can be calculated and the size of a deposit

is predicted (cf. Sandström 2011a).

Earlier studies (Jux 1957, Flodén et al 2001, Eriksson & Cal-

ner 2008) reveal a prograding carbonate ramp system of forced

regressions and transgressive events forming a system of

stacked sequences prograding mainly to the SE and in late Silu-

rian towards S. The major cuclicity of such a sequence forms a

regressive event of subaerial exposure in the upper and mid-

ramp areas. In the uppermost ramp series of stacked stromato-

poriod biostromes may form, separated by truncated surfaces

(Sandström & Kershaw 2002). In Fig. 4, an eventual main se-

quence boundary is marked by the transition from back-reef to

shoal/flank facies. However in the extremely shallow areas,

even minor sea-level fluctuations may cause relevant facies

changes and erosional structures. Or instance, within the MFT

back-reef two levels of probable stromatoporoid biostromes are

evident that most likely are the result of temporary changes sea

-level and exposure. These are also relevant for high purity

limestone production given that the thickness and volumes are

economically adequate.

Concluding Remarks Using dynamic stratigraphy for the purpose of finding high

purity limestone seems to work well. In other parts of the world

this has also been proven (eg. Aigner et al 1999, Pawellek &

Aigner 2001, 2004), there are possibilities to use the method

for other non-metallic sedimentary evironments. Examples of

this include limestone-marl interactions suitable for direct ce-

ment production, clastic deposits for ultra-pure sands, gravel

for concrete and aggregate, claystone, different shales, and

mineralisation due to depositional lags et.c.

Dynamic stratigraphy may be used in both ways; to refine

quarry-planning through deep knowledge in the relation be-

tween different microfacies and to find new deposits by begin-

ning in the basin scale working down to regional and local dy-

namic stratigraphic sequences, establishing local facies types

and microfacies.

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