origin, environmental setting, and tectonic implications...

Colgate University Journal of the Sciences 9

ORIGIN, ENVIRONMENTAL SETTING, AND TECTONIC IMPLICATIONS OF SILURIAN BRECCIAS,

GLACIER BAY NATIONAL PARK, ALASKA

LISA MAYHEW

Abstract In Glacier Bay National Park a Silurian limestone megabreccia, part of the Willoughby Formation

in the Alexander terrane (southeast Alaska), was studied to determine the environment of deposition and tectonic implications. Data were collected through rocks sampled along a transect, quadrat analysis, a clast survey, field observations, and subsequent petrographic analysis. Three distinct clast types were discovered: peloid-dominated limestone, stromatolite boundstone, and cementstone. Fossil assemblages, including aphrosalpingid sponges, Ludlovia, and Sphaerina are found in stromatolite boundstone clasts. Gastropods, brachiopods, corals, Amphipora, Sphaerina, and other microorganisms are present within some of the peloid-dominated clasts.

The different clast lithologies suggest distinct source areas of the clasts; a lagoonal platform environment is suggested for the peloid-dominated clasts while a barrier reef, platform margin environment is postulated for the stromatolite and cementstone clasts. Proportions and sizes of clast types change upsection suggesting that during the Silurian environmental conditions in the Alexander terrane were fluctuating. The onset of tectonic activity associated with the Klakas Orogeny in the Late Silurian may have induced faulting, which initiated brecciation and debris flows of platform and platform margin sediments.

10 Colgate University Journal of the Sciences

Introduction The history of Alaska is a long and complicated story of terranes that have been rafted across

ocean basins and finally accreted onto the edge of the North American continent. The accretion of a

number of geologically dis crete terranes onto the same area forms belts of suspect terranes. The terranes are

suspect because they are pieces of land that have unknown beginnings in unknown places. Terranes move

across the globe as tectonic plates shift; this movement eventually resulted in the formation of the land that

is now Alaska. Geologists have been studying different aspects of the land in order to uncover the

relatively unknown history and origin of the Alaskan terranes (Coney et al., 1980). I conducted my research

in Glacier Bay National Park (Figure 1) which is located in southeastern Alaska in the Alexander terrane.

The terrane is 100,000 km2 in size (Soja and Antoshkina, 1997) and is one of the many fragments that

presently make up the Alaskan panhandle and underlie parts of British Columbia and the Yukon (Butler,

1997).

There have been many theories as to the location of the Alexander terrane during the Silurian.

However, only a few of them are still prevalent today. Previous theories locate the Alexander terrane in

close proximity to western North America, offshore of Australia, or in a mid-oceanic setting, or along the

Uralian Seaway (Jones et al., 1972; Churkin, 1974; Gehrels and Saleeby, 1987a and 1987b; Soja and

Antoshkina, 1997). Soja and Antoshkina (1997) have conducted field research in southeastern Alaska and

the Ural Mountains of Russia studying Silurian fossil-rich deposits. The goal of their research has been to

piece together a story that explains the position of the Alexander terrane during the Silurian through an

integration of evidence with other geologic data provided by fossilized biotas found in Silurian carbonate

deposits. The research has


Figure 1. Map of research locale in Glacier Bay National Park, Alaska. Inset map of Alaska shows location of Glacier Bay.


Figure 2. Map of Drake Island showing the site of research (“Icy Point”). resulted in discoveries of striking similarities in microbial fossil deposits, stromatolites, between the two

locations. This suggests that the Alexander terrane was formed in close proximity to both locations (Soja,

1994a). If this were true, then faunal communication between northwestern North America and northern

Europe during the Silurian would have been possible along the Uralian Seaway.


My research concentrates on breccias composed of stromatolite, cementstone, and peloid-

dominated clasts in southeastern Alaska. Data collected from the limestone megabreccias exposed along

the shore of Drake Island (Figure 2) in Glacier Bay reveal an ancient lagoonal environment and

stromatolite reefs that became fractured and redeposited downslope. The origin and environmental setting

of these Silurian breccias are important pieces of information to help determine where Alaska’s panhandle

originally formed. The data collected indicate that debris flows are responsible for the deposition of the

breccia initiated by storm waves, earthquakes, erosion, overloading and oversteepening of reefs or slopes,

and/or tectonic activity. Correlation of the Willoughby Formation with the Heceta Formation to the south

indicates a tectonically active environment for the Alexander terrane during the Late Silurian – Early

Devonian. Unique organisms found in Laurentia, Baltica, and Siberia (Figure 3) were also found in the

stromatolite clasts of the breccia. Combining tectonic and paleontologic evidence from along the Uralian

Seaway supports the theory of a geographic relationship between the Baltica region of Russia and the

Alexander terrane during the Silurian (Figure 4).


Figure 3. Location of fossils of aphrosalpingid sponges indicating their distribution in the Late Silurian.

Figure 4. Paleogeography of the Alexander terrane showing a model of the Uralian Seaway hypothesis.


Geologic Setting and Paleogeography The Alexander terrane is a sizeable terrane with a relatively undisturbed rock record ranging from

the Late Proterozoic to the Jurassic (Gehrels and Saleeby, 1987a, 1987b; Gehrels et al., 1996). This

continuous record reveals a captivatingly complete history of a volcanic arc. The oldest rocks in the

Alexander terrane are greenschist facies metavolcanic and metasedimentary rocks that belong to the Wales

metamorphic suite and are associated with the Wales orogeny that occurred during the mid-Cambrian to

early Ordovician. These metamorphosed rocks are derived from basaltic to andesitic volcanic rocks,

including pillow flows, breccia, and tuff and volcaniclastic strata associated with greywacke, mudstone,

and limestone (Gehrels and Saleeby, 1987a, 1987b). That the Alexander terrane was formed in a volcanic

arc environment associated with a convergent plate margin is evidenced by similarities with rocks found in

modern volcanic arc complexes (Gehrels and Saleeby, 1987a, 1987b). The Wales orogeny interrupted the

volcanic, plutonic, and sedimentary processes that were associated with the arc environment. However,

volcanic and sedimentary activity resumed soon after the orogeny ended and continued until mid Early

Silurian (Gehrels and Saleeby, 1987a, 1987b)

The Descon Formation represents Ordovician to mid Early Silurian rocks and is the most widely

distributed formation in the Alexander terrane. The formation is at least 3,000 m thick and is composed of

five main units (Eberlein and Churkin, 1970). The unit that covers the most area is greywacke, mainly a

volcanic sandstone, interbedded with a banded mudstone. The second most important unit is composed of

conglomerate and sedimentary megabreccia. The clasts in these beds vary from places where they are all

volcanic to places where the conglomerate is polymictic. Clast types include varicolored chert, limestone,

felsic volcanics, and granite to gabbroic rock types. The third unit is black chert and siliceous shale that is

interbedded throughout the formation with the other units. The shale contains graptolites that are used for

dating and correlating beds. The fourth unit consists of basaltic volcanic rocks in a number of different

forms including pillow flows, flow breccia, volcanic conglomerate, and agglomerate. Dating of these beds

shows that volcanism was occurring throughout the Early Ordovician to the Early Silurian. The fifth unit is

a quartzo-feldspathic sandstone that is often found in association with the greywacke and banded mudstone.

The Descon conformably underlies the Heceta Formation in most localities, although in some areas it

unconformably underlies the Karheen, a Devonian deposit (Eberlein and Churkin, 1970). The rocks of the


Descon are much less deformed and metamorphosed than those of the underlying Wales suite (Gehrels and

Saleeby, 1987a, 1987b).

The stromatolite breccias I investigated are found in the Willoughby Formation which was

deposited during the Late Silurian in the area that is now Glacier Bay, a glaciated fjord that is 800 km north

of Prince of Wales Island. Some parts of the Willoughby Formation in Glacier Bay National Park have

been correlated with the Heceta Limestone based on the co-occurrence of certain unique sponges,

microbes, and other fossils (Soja, 1999, pers.comm.; Gleason, 1998). The Willoughby consists of

limestones, primarily peloid-dominated grainstones, stromatolites, and breccias. The limestone generally

has a fine to medium grain size although in some areas it has been intensely metamorphosed and is a

coarse-grained marble (Seitz, 1959). In the area in which I worked the beds had an average thickness of

10-25 cm. (Figure 5).


Figure 5. IP-99 bedding looking north.

Although, throughout the formation the beds vary from 5 cm to many meters thick and most commonly

they are 15 cm -1 m thick (Rossman, 1963). In its thickest exposures the Willoughby has been calculated

to be about 1,500 m thick (Rossman, 1963).

The Heceta Limestone, which is correlative to the Willoughby Formation in Glacier Bay National

Park, conformably overlies the Ordovician to mid Early Silurian Descon Formation to the south on Prince

of Wales Island. Much of the limestone formed in a shallow marine environment (Eberlein and Churkin,

1970; Soja and Antoshkina, 1997), while some formed in deep-marine environments along the foreslope of

the arc shelf. Debris flows and turbidites consisting of carbonates make up many of these deposits (Soja,

1991,1994a). The limestone is mainly massive, thick-bedded, and fine-grained. Near the middle of the

formation there are locally thick deposits of polymictic conglomerate, limestone breccia, sandstone, and

argillaceous rocks. Fossils of corals and brachiopods as well as stromatoporoids, sphinctozoan sponges,

brachiopods, gastropods, rare bryozoans, calcareous algae, trilobites, and calcified microorganisms are

found in the Heceta (Soja, 1991,1994a).


These layers must have been deposited after the cessation of Descon-age volcanism because they

are not interbedded with volcanic rocks. The Heceta is not present everywhere in the Alexander terrane

because in some areas intrusive rocks are the only representatives of this time period (Gehrels and Saleeby,

1987a, 1987b). Coeval with the termination of the Heceta Limestone was the Klakas orogeny, which began

after the middle Early Silurian and ended by the middle Early Devonian (Gehrels and Saleeby1987b). This

orogeny is recorded in the unconformity that lies, in some areas, between the Silurian and Devonian strata.

The orogeny is also recorded in incipient stages of the Silurian conglomerate strata (Gehrels and Saleeby,

1987a, 1987b).

The upwardly fining strata of the Karheen Formation formed in the Early Devonian and

conformably overlie Silurian rocks in some areas on Prince of Wales Island. The base of the Karheen

consists of a cobble to boulder size conglomerate with Ordovician and Silurian volcanic and plutonic clasts

(Gehrels and Saleeby, 1987a, 1987b). This indicates a time of high topography that would allow for the

erosion of the older rocks before and during the deposition of the Devonian rocks. Higher in the formation

the conglomerate grades to sandstone, siltstone, and limestone, which represent intertidal to shallow-marine

environments. Above the limestone laminated and layered mudstone and black shale are found. The

evidence suggests that these strata were deposited in distal regions of a submarine fan (Gehrels and

Saleeby, 1987b). A deep water, low energy environment would explain the observation of laminated and

layered mudstone and black shale. This suggests that subsidence or a relative rise in sea level must have

followed the orogeny (Gehrels and Saleeby, 1987a, 1987b). The volcanic rocks, dacites, basalts, and

andesites, indicate the presence of volcanoes which confirms the theory of an island arc area (Gehrels and

Saleeby, 1987b). The Karheen in some areas has a minimum thickness of 1800 m although this may be a

localized anomaly. In many other areas the minimum thickness is thought to be about 1,000 m (Eberlein

and Churkin, 1970).

Through interpreting the origin and paleoenvironment of the breccias found in the Willoughby

Limestone, and understanding the changes that cause deposition of the overlying strata, hypotheses

concerning the paleogeographic location of the Alexander terrane may be supported or refuted. One of the

original hypotheses by Monger and Ross (1971) postulates that the terrane was located along the Sierra-

Klamath region of the southwestern United States because of similar faunas found in Silurian and Permian


strata in Alaska and northern California (Gehrels and Saleeby, 1987b; Gehrels et al., 1996). This theory

was initially supported by similar geologic records (Jones et al., 1972), paleomagnetic data (Van der Voo et

al., 1980), tectonic evidence, and biogeographic analyses (Gehrels et al., 1996). It has since been

discovered that the tectonic histories of the Alexander terrane and the Sierra-Klamath region (Northern

Sierra and Klamath terranes), during the time period in question, are vastly different. The periods of

orogenic activity, as recorded in each terrane, occurred at disparate times (Gehrels and Saleeby, 1987b).

According to Gehrels et al. (1996), detrital zircon ages are vastly different as well. There are also many

petrologic differences between the two locations. These disparities include different types of rocks, i.e.

sedimentary in the Alexander terrane vs. volcanic in the Klamath terrane, being deposited during the same

time period (Gehrels and Saleeby, 1987b).

A paleo-position near northwestern North America is one hypothesis that envisions the continuous

opening and closing of ocean basins to explain the movement and tectonism that is recorded in the terrane

(Churkin and Eberlein, 1977). There are paleobiogeographic similarities between the Alexander terrane

and part of southwestern Alaska’s Farewell (formerly the Nixon Fork) terrane, but not with many rocks in

Alaska, northwestern Canada, and the western U.S (Soja, 1994a; Gehrels et al., 1996). This hypothesis is

also supported by paleomagnetic data but is refuted by detrital zircon data and the timing of

tectonic/orogenic activity (Gehrels et al., 1996).

The paleo-Pacific margin of Australia-Antarctica has been suggested as another likely

paleoposition for the Alexander terrane in the early-mid Paleozoic. This theory is mainly supported by

concurrent timing of arc-type magmatism and orogenic activity (Gehrels and Saleeby, 1987b).

Paleomagnetic data (Van der Voo et al., 1980; Bazard et al., 1995), and detrital zircon data may support

this hypothesis as well. Most of the zircons dated revealed Ordovician to Silurian ages that can be

explained by the presence of plutonic rocks of those ages present in the Alexander terrane. However,

zircons with ages of 1.0 to 3.0 Ga were found in the Karheen Formation and indicate a foreign source

because no rocks of that age are found in the terrane itself. The oldest rocks in the Alexander terrane are

595 Ma. These Precambrian zircons suggest that the terrane was located near a continental craton in the

Late Silurian to Early Devonian, which would contain minerals spanning those ages that could become

resedimented as part of the Karheen Formation during the Early Devonian. The dates recorded in the


zircons correlate with rocks of the same ages that are found in Australia-Antarctica and the western Baltic

shield (Gehrels et al, 1996). The main bulk of evidence that refutes the Australia-Antarctica hypothesis is

the biogeographic data (Soja, 1994a; Soja and Antoshkina, 1997; Gehrels et al., 1996; Bazard et al., 1995).

One of the newest suggestions for the Alexander terrane’s paleo-position was put forth by Soja

(1994) and Soja and Antoshkina (1997) and places the terrane somewhere between northwestern North

America and Baltica (northern Europe) in the Late Silurian. They postulate that the terrane was located

along the Uralian Seaway, which existed between the coasts of northern North America (Laurentia),

northern Europe (Baltica), and Siberia during the Late Silurian (Soja and Anthoshkina, 1997, figure 3).

This hypothesis was originally suggested because the paleontologic similarities between the terrane and the

Ural Mountains of Russia correlate with options available from other geologic data (Bazard et al., 1995;

Soja and Anthoshkina, 1997). Paleomagnetic data, concurrent tectonic/orogenic activity, and the feasibility

of the Baltic Shield as the source of detrital zircons all support this hypothesis (Bazard et al., 1995; Gehrels

et al., 1996; Soja and Antoshkina, 1997). More detailed studies in Baltica, the Canadian Arctic, Russia,

Siberia, and Australia will help to support or refute this idea.

Silurian stromatolite reefs found in the Alexander terrane in the Heceta and Willoughby

Formations contain some of the main evidence for a paleo-position along the Uralian Seaway (Soja, 1991,

1993, 1994a, 1994b; Soja and Antoshkina, 1997). Aphrosalpingid sponges (Figure 6), including

Aphrosalpinx textilis, are found only in stromatolite reefs and reef-derived clasts in the Alexander terrane

(Soja, 1991).


Figure 6. Aphrosalpingid sponges in stromatolite boundstone from sample IP-5.9W-99. Scale = 1 mm. Stratigraphic top is towards top of page.

The presence of aphrosalpingids is significant because the only other places they have been found are in

parts of Russia and southwestern Alaska (Soja, 1991). Therefore the Alexander terrane had to be located

somewhere where biotic communication would be possible between itself and Baltica, Siberia, and

northwestern North America. The Uralian Seaway was a likely means of biotic communication (Soja and

Anthoshkina, 1997). The new data compiled in this report, concerning breccias of the Willoughby

Formation, support the Uralian Seaway model.

Materials and Methods Samples of rocks from the Willoughby Formation were collected from the western shore of Drake

Island in Glacier Bay National Park. Thirty-nine samples (Figure 7) were collected from the breccia

exposure at “Icy Point”. Sampling was done along a 9.3+ meter stratigraphic transect at 0.4-1.0 meter


intervals in order to obtain an accurate representation of the section. The total thickness of the outcrop is

unknown because the transect culminated on an obliquely polished, glaciated surface that had no

discernible bedding planes. Spot samples were acquired at sites off of the transect for a wider

representation of exposed lithologies. Samples were also taken from specific clasts on the obliquely

polished slab at 9.3+ m in order to study the different megaclast types (Figure 8). All of the samples were

oriented and labeled to show their stratigraphic top. The hand samples were cut into billets using a rock

saw, and 41 thin sections were prepared. The thin sections were examined under the PetroScope with

further examination conducted on a petrographic microscope. The contents (grains, matrix, cement) of

each thin section were identified, the relative abundance of the contents was determined, and the rocks

were classified (Appendix 1). Dunhams’s method of classification was employed except for a new term,

cementstone, which is introduced to describe clasts composed solely of fibrous, radiating fan, and/or blocky

cements.

Four 1 m2 quadrats were constructed in order to obtain a representative record of the site. In total,

seventy-five randomly chosen clasts contained within the quadrats were measured and classified (Tables 1,

2, and 3). The quadrats were located on representative bedding planes at 1.3 m and 3.7 m above the base,

and two were located at 9.1 m above the base of the section.


Figure 7. Stratigraphic column of rocks exposed at “Icy Point” showing upward changes of size and shape and location of collection samples.


Figure 8. Looking seaward from bedding plane 9.1 m above the base.

Table 1 Quadrats 1 and 2 Data

Quadrat 1 - IP-1.3-99, small lime mud clasts w/ scattered brachs Quadrat 2 - IP-3.7-99, large laminite clast is partially included in quadrat

Quadrat 1 Quadrat 2 clast # Length

(cm) Width (cm) clast # Length

(cm) Width (cm)

1 3.5 0.5 1 3.2 0.8 2 5.5 1 2 7.5 1 3 3 0.6 3 2.5 1.4 4 5 1.1 4 6 3.1 5 3.9 1.5 5 10 2 6 5 1.9 6 8 3.5 7 4 0.7 7 4.5 2.3 8 2.6 1.5 8 11.3 3.6 9 2.7 1.1 9 3.5 1.3

10 2.4 0.9 10 5.5 1.5 11 4.8 1 11 6 1.7 12 4.8 0.7 12 5.4 1.8 13 10.5 1.4 13 6.5 1.7 14 16 5 14 4.4 1.8 15 13.5 6.5 15 7.2 0.6


16 4.3 1 16 9.9 1.5 17 5 4 17 6.6 2 18 4 2 18 2.2 1.4 19 3 1.2 19 4 2.7 20 9 2.5 20 3 1 21 12.5 1.9 21 4 0.5 22 6 2 22 5 1.1 23 3.8 2.6 23 4.5 3.5 24 5.5 2.6 24 3.3 1.1 25 3.5 3.2 25 3.6 2.6 w/snail

Table 1. Description of clasts located with 1m2 quadrat.

Table 2 Quadrat 3 Data

Quadrat 3 - composed of two separate 1m x 1m quadrats of IP-9.1-99

99 (3a and 3b)

clast # Length (cm)

Width (cm) Clast Type

1 9 6.4 boundstone 2 6.9 4.5 boundstone 3 5 4.4 mudstone 4 11 7.5 boundstone 5 1.6 1.3 mudstone 6 7 3.8 boundstone 7 4.7 2.6 mudstone 8 3.7 1.9 mudstone 9 4 3.4 boundstone

10 38 23 part of 30 x 38 boundstone 11 9.3 4 mudstone 12 3 2 mudstone 13 3.8 2.1 boundstone 14 4 4 mudstone

Quadrat 3b - on edge of boundstone megaclast, includes 27 x 31 cm part of 45 x 31cm boundstone clast

15 26.1 23.5 boundstone 16 6.3 0.7 mudstone 17 15.2 14.9 boundstone 18 9.4 3.7 mudstone 19 6.5 2 mudstone 20 4 1.2 mudstone 21 3.7 3.2 boundstone 22 1 1 mudstone 23 3.1 2.4 mudstone 24 1.1 0.8 mudstone 25 5.5 4.2 boundstone

Table 2. Description of clasts within 1m2 quadrats.

Table 3 Average Clast Sizes


Mudstone Cementstone Boundstone Length (m)

Width (m)

Length (m)

Width (m)

Length (m)

Width (m)

Quadrat 1 0.0575 0.0194 n/a

n/a

n/a

n/a

Quadrat 2 0.055 0.018 n/a

n/a

n/a

n/a

Quadrat 3a + 3b

0.045 0.023 n/a

n/a

0.092 0.074

Clast Survey

1.06 0.5 1.67 1.12 1.8 1.26

Table 3. Average size of clasts within quadrats.

Tracings were made of a representative area of lime mudstone clasts on bedding plane 1.0 m above the base

(Figure 9). A laminite mud clast containing brachiopods was traced (Figure 10), as was a representative

stromatolitic clast from bedding plane 9.3 m above the base. The tracings are accurate portrayals of

relative size, sorting, and abundance of different clasts in the breccia. Working at mid to low tide when

there was more bedding surface exposed, a clast survey was conducted on bedding planes 9.3+, in order to

obtain an accurate representation of the trends and variety within the breccia. The area covered was 29

meters from north to south and 10 meters from east to west. Only clasts larger than 0.5 m in one dimension

were measured. This constraint was imposed to limit the number of clasts examined to a reasonable

amount. Data recorded included minimum length, width, and thickness, rock type, shape, and surrounding

material (Table 4).


Figure 9. IP-1.0-99. Tracing of typical lime mudstone clasts.


Figure 10. Tracing of large laminite clast with concentration of brachipod fossils.


Table 4 Clast Census Data Clast #

Minimum Minimum Minimum Rock Type Shape Surrounding Material

Length Width Depth 1 1.2 0.75 no data cementstone; small

areas subround;

dark muddy matrix

of boundstone; small

embayed

with abundant

cavities outline smaller clasts 2 2.9 1.25 0.05 cemenstone;

boundstone; subround/

s.o.s; large clast

Fistulella round 3 5.1 3.6 0.3 large cementstone subang

ular/ s.o.s; large clast

cavities; laminated subround fibrous cements; boundstone; Fistulella 4 1.4 1.3 0.1 tectonized clast;

dis- subangular

s.o.s

rupted cements and boundstone 5 2 1.4 0.1 boundstone;

Fistulella; subround

large lime mud clasts;

rare aragonite needles cemented mud clasts 6 1.5 0.35 no data lime mud clast;

gastropod subangular

cemented mud clasts

wackestone; faint btw clasts 5,7,8 lamination 7 1.7 1.15 0.3 boundstone;

Fistulella; subangular/

cemented mud clasts

small cavities; sponges

subround

clast 6

8 1.95 0.9 no data lime mudstone;

snails; subround/

cemented mud clasts

one coral; fenestrae

round

9 0.75 0.4 no data boundstone;

Fistulella; subangular

cemented mud clasts

sponges; possible coral or large sponge 10 0.55 0.5 no data cementstone subroun

d cemented mud clasts


11 1.3 1.3 no data possible brachiopod

subangular/

cemented mud clasts

boundstone; sponges;

subround;

large Fistulella?; branching

embayed

Fistulella?; cementstone;

outline

cavities w/ aragonite needles and fibrous cements 12 0.7 0.5 no data cementstone subroun


13 1.6 1.4 0.1 boundstone;


cemented mud clasts;

small cavities clast 12 14 0.65 0.3 no data lime mudstone;

fibrous angular/ cemented mud clasts;

cement filled cavities with

subangular

clast 15

sponges-aphro? 15 0.55 0.25 no data cementstone subang

ular/ cemented mud clasts

subround 16 0.65 0.35 no data boundstone; large subroun


cavities; Fistulella; 17 1 0.85 no data boundstone; cross subroun


section through domes appear to include radiating aragonite needle fans; no Fistulella?; sponges 18 0.6 0.575 no data boundstone; same

as 17; subangular/


sponges; florets; no

subround

boundstone clast 17

Fistulella? 19 0.6 0.45 no data bounstone;

Fistulella subangular


clast 18 20 0.55 0.25 no data laminated lime

mudstone; angular cemented mud clasts

finestrae; one clam 21 1.25 0.7 no data boundstone with subroun cemented mud clasts


larger d concentric dones; cavity; interlaminated aragonite fans; sponge; no Fistulella? 22 8.25 5.6 no data boundstone;

sponges; subangular/


cavities; aragonite fans;

subround

cement fringe

Fistulella? 23 1.05 1.05 no data boundstone;


cemented mud

aragonite needle fans clasts 24 0.75 0.6 no data boundstone;

Fistulella; subangular

muddy matrix

concentric microbial domes; aragonite fans 25 0.85 0.45 no data cementstone subang

ular muddy matrix

26 0.95 0.65 no data boundstone;

concentric subangular

muddy matrix;

aragonite florets; sponge other clasts 27 1.55 1.2 no data boundstone;

columnar subangular/

muddy matrix

microbial growths; large

subround

cavities filled with aragonite fans; Fistulella; sponges 28 0.8 0.55 no data boundstone;

Fistulella sub angular

muddy matrix

29 0.9 0.6 no data lime mudstone;

gastropods ? muddy matrix

and/or brachiopods; cavity filled w/ fibrous and blocky cements 30 3.1 1.85 no data cementstone;

cavities with subround

muddy matrix

blocky cements; large botryoids 31 0.75 0.6 no data 1/3 lime

mudstone/coral + angular muddy matrix

snail wackestone; 2/3 concentrated cavities; fibrous cements; one aragonite fan 32 0.8 0.55 no data dark lime angular/ muddy matrix


mudstone; large calcite stringers;

finestrae; sub angular

peloidal?; brachiopods?; snails? 33 1.65 1.45 no data same as 2/3 of

clast 31; ? muddy matrix with

good evidence of Fistulella; larger clasts (buried finger like microbial growths larger clasts edges) 35 4.95 3.45+ no data cementstone;

displaced by angular little muddy matrix;

(buried fault other clasts edges) 36 1 0.55 no data cementstone;

same as 35 subangular/

muddy matrix

subround 37 2.2 0.85 no data boundstone;

cavities; subangular/

muddy matrix;


other clasts

37 2.2 0.85 no data boundstone; cavities; Fistulella; 38 5.05 2.4 no data boundstone;

concentrically subround;

muddy matrix

developed domes; dark/light

embayed

lamination; rare Fistulella;

edges

like stacey's basal stromatolites; beautiful!! 39 1.15 0.75 no data boundstone;

stretched subround;

muddy matrix

cavities; abundant Fistulella?

ragged

margins 40 1.9 1.3 no data cementstone;

florets, well subround/

muddy matrix

defined due to microbial

round

growth?; cross section through outward radiating fans of aragonite 41 1.5 0.9 0.25 cementstone subang

ular/ muddy matrix


Subround Table 4. Description of megaclasts examined in the clast survey conducted on bedding plane 9.3+.

Overview photographs of the field site were taken as well as photographs of lithologic features

along the transect. Close-ups were taken when the rock was both dry and wet in order to accentuate certain

aspects. Many photographs were taken from a ladder to obtain a bird's eye view of the breccia. Each clast

examined in the clast survey was also photographed.

Results Data were gathered from field observations along a measured transect and the subsequent

examination of 41 thin sections from hand samples. Observations from 1 m2 quadrats constructed on the

outcrop, a census of clast sizes, shapes, and composition, along with the transect and thin section data were

used to recognize lithologic trends and form the basis for paleoenvironment and tectonic interpretations.

In the field, rocks and clasts were characterized according to their observable features. This

technique resulted in three categories of rock type: lime mudstone, boundstone, and cementstone.

Mudstones were identified by the blue-grey appearance of limestone often with white laminations (Figures

11 and 12). Boundstones were identified by concentric circular to ovoid and dark and light layering

(Figures 13 and 14).


Figure 11. Laminated mudstone clast (wet). Sample IP-3.7-99. Ruler = 15 cm.

Figure 12. IP-9.3+-99. Laminite mudstone clast #20 (clast survey). Ruler = 15 cm.


Figure 13. IP-9.3-99. Stromatolite clast #1 (clast survey) showing size and shape of clasts. Ruler = 15 cm.

Figure 14. Close up of concentric microbial layers of stromatolite clast #7 (clast survey) with sponge in center at IP-9.3+-99.


Cementstones were most often identified by fan-like, fibrous and/or blocky cement structures (Figures 15 and 16).

Figure 15. Cementstone clast #40 (clast survey) showing concentric layering of radiating fan cements at IP-9.3+-99. Ruler = 15 cm.

Figure 16. Close up of radiating fan cements of clast #27 (clast survey) at IP-9.3+-99. Ruler = 15 cm.

The forty-one thin sections examined were classified into six main groups. The most common

class is peloid-dominated clasts of mud/wacke/pack/grainstones which were originally classified as

mudstones in the field. Their specific classification varies according to the proportion of cement to matrix


(Figures 17-20). They generally contain 28-85% peloids, 10-55% calcite cement, and 3-45% micrite

matrix.

Figure 17. IP-3.5B-99. Photomicrograph of peloidal mudstone clast from sample IP 3.5B-99 showing relative proportions of peloids, cement, and micrite. Scale = 2 mm. Stratigraphic top is towards top of page.

Some of these clasts also contain 2-10% Amphipora (Figure 21), 2-10% brachiopods (Figure 22), 5-8% corals, and 1-5% microorganisms. Peloidal/gastropod packstones (Figure 23) are characterized by 25-40% gastropods, 20-35% peloids, and 30% calcite cement. Stromatolite boundstones (Figure 24) are characterized by 60-99% microbial layering with mainly aphrosalpingid sponges (Figure 6), microorganisms, and calcite cement comprising the remaining 1-30%. Cementstones are characterized by 92-


Figure 18. Photomicrograph of peloidal wackestone clast from sample IP-2.5-99 showing relative proportions of peloids, cement and micrite.

Scale = 2 mm. Stratigraphic top is towards top of page.

Figure 19. Photomicrogaph of peloidal packstone clast with lumps from sample IP-4.3-99. Scale = 2 mm. Stratigraphic top is towards top of page.


Figure 20. Photomicrograph of peloidal grainstone clast offset by small fault and surrounded by blocky calcite cement from sample IP-8.7W-99. Scale = 2 mm. Stratigraphic top is towards top of page.

Figure 21. Photomicrograph of Amphipora in peloidal packstone from sample IP-4.5B-99. Scale = 2 mm. Stratigraphic top is towards top of page.


Figure 22. Photomicrograph of cluster of brachiopods in peloidal packstone from sample IP-3.5B-99. Scale = 2 mm. Stratigraphic top is towards right side of page.

Figure 23. Photomicrograph of gastropod s in peloidal packstone from sample IP-S1a-99. Scale = 2 mm. Stratigraphic top is towards left side of page.


Figure 24. Photomicrogaph of stromatolite boundstone with replacement quartz from sample IP-9.5E-99. Scale = 1 mm. Stratigraphic top is towards top of page.

100% calcite cement with quartz blebs comprising the remaining 0-8%. Other categories such as peloidal/Amphipora packstone and peloidal/brachiopod wackestone are used to characterize clasts with an abundance of the respective organisms.

The outcrop along the 9.3+-meter transect is a breccia of peloid-dominated, stromatolite, and cementstone clasts. The breccia is thinly to moderately bedded (Figures 25 and 26); the beds are more continuous higher in the section whereas lower in the section they are periodically interrupted by gravel beach deposits (Figure 27). The lower 7.0 m of deposits along the transect are characterized by a predominance of clasts surrounded by cements, which generally range in size from 2.40-13.5 cm long and 0.5-6.5 cm wide, (Figure 28).


Figure 25. Bedding at “Icy Point” looking north from 8.5-9.3 m.

Figure 26. Bedding at “Icy Point” from 0.0-0.3 m.


Figure 27. Bedding at “Icy Point” looking north from 4.5 m.

Figure 28. Typical lime mudstone clasts at IP-0.7-99.

Most of these clasts are peloidal wackestones and packstones with the exception of one larger

stromatolite clast and three larger laminite clasts. Thin sections of clasts from these lower beds generally


contain 30-45% peloids, 10-50% calcite cement, and 3-25% micrite (Appendix 1). They typically contain

very few fossils ranging from 2-10% Amphipora (Figure 21), 2-5% brachiopods (Figures 22 and 29) and/or

gastropods (Figures 23 and 30), 5-8% corals, and 1-2% microorganisms (Figure 31). Large concentrations

of whole gastropod and brachiopod fossils are present in the few larger laminite clasts (>0.5 m in one

dimension) in the lower beds.

Upsection the deposits at 7.0-9.3 m above the base comprise larger fenestral lime mudstone clasts

in well-bedded units. A major depositional change along the transect occurs at 9.3 m above the base

marked by the first abundant stromatolite boundstone megaclasts (> 0.5 m) (Figure 13). Stromatolite clasts

occur lower in the section but they are not abundant or generally as large as those upsection. From 9.1 to

9.3+ m, where the section forms an obliquely polished plane, deposits are characterized by stromatolite and

cementstone megaclasts surrounded by smaller clasts of varying compositions (Figure 8).

Quadrat 1 (Table 1) was located on bedding plane 1.3 m and consists of small peloidal wackestone

and packstone clasts with scattered brachiopod fossils. The length of the clasts ranges from 2.4-13.5 cm

and the width of the clasts ranges from 0.5-6.5 cm. Quadrat 2 (Table 1) was located on bedding plane 3.7

m and included small peloidal wackestone and packstone clasts and a large laminite clast characterized by

fine-grained micrite and fenestrae. The length of these clasts ranges from 3.0-11.3 cm. The width of the

clasts ranged from 0.5-3.6 cm. Clasts in quadrats 1 and 2 are similar in size and composition (see Table 3

for averages). On bedding plane 9.1 m, 14 clasts were


Figure 29. IP-6.1-99. Brachiopods and gastropods in laminite clast. Ruler = 15 cm.

Figure 30. IP-5.2-99. Gastropods and brachiopods in laminite clast. Ruler = 15cm.


Figure 31. Photomicrograph of microorganism (Sphaerina) in peloid-dominated clast. Scale = 1 mm. Stratigraphic top towards top of page.

measured in quadrat 3a, and 11 clasts were measured in quadrat 3b (Table 2). Eleven

stromatolite boundstone clasts and 14 peloid-dominated clasts within the two quadrats were examined. The

average size and size range of the boundstone clasts were larger than the average size and size range of the

“mudstone” clasts (Table 3).

A clast survey was conducted on the obliquely polished upper bedding planes (9.3+ m) to

determine trends and variety within the breccia. The survey consisted of 41 clasts with one dimension

larger than 0.5 m (Table 4). The clasts were measured for minimum length, width, and thickness, shape,

composition and type of surrounding material. The boundstone clasts (Figures 13 and 14) were the largest

(Table 3), ranging in length from 0.6-8.25 m while the width ranged from 0.35-3.0+ m. Cementstone clasts

(Figures 15 and 16) varied in length from 0.55-4.95 m and in width from 0.25-3.45+ m. Lime mudstone

and peloidal wackestone clasts (Figures 11 and 12) varied in length from 0.55-1.95 m and in width from

0.25-0.9 m. Six different categories were used for classifying the shape of clasts: 7.3% of clasts were

subround/round, 29.2% of clasts were subround, 24.4% of clasts were subangular/subround, 22.0% of

clasts were subangular, 4.9% of clasts were angular/subangular, 7.3% of clasts were angular, and 4.9% of

clasts were not classified because the edges were obscured. Most clasts ranged from subangular/subround


to angular (Figure 32). Of the clasts surveyed 46.3% were boundstone, 24.4% were cementstone, 12.1%

had aspects of both cementstone and boundstone, 14.6% were lime mudstone or wackestone, and 2.4%

consisted of lime mudstone and cementstone. All clasts are surrounded by cemented peloidal clasts and

other large cementstone and/or stromatolite boundstone clasts (Table 4).

Figure 32. IP-9.3-99. Close-up of stromatolite clast #1 (clast survey) showing shape and definition of edges.

Discussion The data collected at “Icy Point” reveal significant patterns and changes that

occur in the breccia facies of the Willoughby Formation at this location. Peloidal limestones, stromatolite

boundstones, and cementstones comprise the primary clasts present along the transect. Peloid-dominated

clasts were the most abundant clast type. Fossil assemblages, including gastropods (Figures 23 and 30),

brachiopods (Figures 22 and 29), corals, Amphipora (Figure 21), Ludlovia (Figure 33), and microorganisms

(Figure 31), are a common constituent of larger peloidal clasts. Stromatolite boundstone clasts (Figure 3)

are composed of concentric circular or ovoid microbial laminae with margins that in cross-section generally

appear scalloped. Aphrosalpingid (sphinctozoan) sponges (Figure 6) are found in association with 7% of


the stromatolite clasts (Appendix 1). Cementstone clasts, recognized only in the upper stratigraphic unit

(>9.3 m above the base of the section), are composed of fibrous, blocky, and radiating calcite/aragonite

cements (Figure 34) with secondary quartz and a neomorphosed (recrystallized) fabric.

The presence of well-defined bedding and a predominance of small peloid-dominated clasts

(generally 1.94 cm by 5.75 cm) (Figures 5 and 28) typifies the lower 9.2 m of the section. Large fenestral

peloidal clasts (27 cm by 87 cm) (Figures 11 and 12) are rare throughout the transect area. Higher in the

transect the characteristics of the breccia change significantly: at 9.3 m stromatolite clasts found rarely in

the lower 9.2 m become the dominant clast type and distinct bedding planes are no longer recognizable,

mainly because of glacial erosion and polishing (Figure 8). Stromatolite and cementstone megaclasts (on

average 1.71 m by 1.14 m) in a matrix of peloid-dominated clasts that are characteristic of the lower

section, typify the upper units exposed at the site.

Figure 33. IP-9.2W-99. Photomicrograph of Ludlovia in stromatolite boundstone clast. Scale = 2 mm. Stratigraphic top towards top of page.


Figure 34. Photomicrogaph of radiating fan and block cements from sample IP-3.5A-99. Scale = 2 mm. Stratigraphic top towards top of page.


Major changes in paleoenvironmental conditions on Drake Island during

the Late Silurian are interpreted based on the field and petrographic evidence as well as comparison with

the correlative unit, the Heceta Formation, exposed 800 km to the south (Soja et al., 1997). The polymictic

nature of the breccia investigated at “Icy Point” indicates that there were at least two distinct source areas

of the brecciated material (Figure 35). The peloidal limestones clasts that dominate in the lower beds

suggest a lagoonal source area on a shelf margin platform environment. Amphipora (Figure 21) is a

lagoonal indicator (Soja, 1990) and present in 31.7% of clasts. Gastropod concentrates (Figures 23 and 30)

are found in 19.5% of limestone clasts, low-diversity brachiopod assemblages (Figures 22 and 29) in 26.8%

of limestone clasts, and rare corals in 4.9% of limestone clasts, are further evidence of a lagoonal source

area (Flynn et al.,1998; Gleason, 1998). Microbial coatings around fossils indicate a shallow marine

environment (Soja, 1990); many of the gastropod and coral fossils and calcareous algae at the site

possessed microbial coatings.

Figure 35. Cross-section of paleoenvironments believed to have existed near Drake Island, GBNP.

Reconnaissance work conducted downsection of the megabreccia studied revealed deposits of

alternating edgewise breccia, fenestral dolomite/limestone, and finely laminated dolomite/limestone. These

older deposits are the probable source of fenestral limestone clasts found within the megabreccia. The

abundant, large stromatolite boundstone megaclasts and cementstone megaclasts indicate the presence of an

extensive stromatolite reef similar to that which forms part of the Heceta Formation (Soja, 1990). Evidence

from underlying facies supports the presence of stromatolite reefs. Extensive stromatolite reef deposits are

found downsection on Drake Island in deposits that underlie the megabreccia studied (Joyce, 1999, pers.

comm.; 1999 pers. observation).


Rare stromatolite clasts found lower in the stratigraphic section, before they become abundant at

9.0 m above the base, suggest that the microbial reefs were a persistent but not continuous feature at the

platform margin and initially experienced only relatively minor, localized collapse. The stromatolite reefs

on the platform margin would have acted as barrier reefs, creating the conditions necessary for the lagoonal

environment that is in evidence by the peloid-dominated clasts and correlating fossil assemblages.

Characteristics of breccias vary according to the method of formation of the breccia. There are

three common methods of breccia formation with features similar to those in the Willoughby Formation.

Collapse due to evaporite dissolution (Lavoie and Sangster, 1995), tempestites and/or turbidites (Jones and

Desroches, 1992; Tucker and Wright, 1990), and debris flows (Conaghan et al., 1976) can all result in the

formation of breccias and megabreccias.

One common method of formation of breccias is dissolution of evaporite deposits that results in

karst collapse. Lavoie and Sangster (1995) described Lower Carboniferous karstic breccias in Nova Scotia.

The Pembroke breccia was determined to be a collapse breccia based on numerous lines of evidence

including: (1) dissolution features apparent on the surface of limestone clasts; (2) oxidized rims around

carbonate fragments; (3) sedimentary trends such as coarsening- and fining-upward; (4) common red color

of the surrounding carbonate matrix; (5) differing roundness of clasts suggesting different sources and/or

transportational histories for different clasts; (6) red siliciclastic infills in some pores; and (7) the likely

meteoric origin of cements associated with infills. Jadoul et al. (1991) describe other characteristics

including vadose calcite cements, internal sediments, and dissolution structures. None of these attributes is

apparent in the megabreccia studied on Drake Island, which suggests that the breccia was not formed by

evaporite dissolution and collapse.

There are many processes through which material can be moved downslope and redeposited.

Mechanisms of redeposition that produce well-sorted material and discrete, planar surfaces (Johns, 1978)

are not considered as possible mechanisms for deposition of the Willoughby breccia’s angular, poorly

sorted clasts. Tempestites are the result of storm waves and currents that cause resedimentation and

basinward transport of shallow marine material (Jones and Desroches, 1992). Tempestite beds range from

interlaminated grainstones and mudstones which are interbedded with bioturbated wacke/mudstones. Other

features of tempestites include hummocky cross-stratified bedding, scored bases, grading, often fining


upward, and interference ripples as well as skeletal layers that overlie erosional sequences (Jones and

Desroches, 1992). Again none of these features is apparent in the Willoughby Formation breccia, which

suggests that it was not deposited by tempestites.

Turbidity currents are an effective method of moving material downslope, which result in the

deposition of turbidites. Turbidity currents move sediment that is supported by fluid turbulence from

escaping pore-fluid (Tucker and Wright, 1990). Turbidites are generally coarse to fine-grained deposits

containing mud to cobble size material. Flat-lamination, cross-stratification, scour structures, and inverse-

or normal-grading are common characteristics of turbidite deposits (Tucker and Wright, 1990). These

features are not apparent in the Drake Island breccia and therefore turbidites are not postulated as the

method of deposition of the breccia. Debris flows are another means of moving material downslope. They

generally comprise a matrix of mud and water that can carry clasts that are denser than the bulk density of

the flow (Coniglio and Dix, 1992; Scoffin, 1987). The flows exhibit plastic-like behavior while moving

along a slope with reduced shear strength (Spence and Tucker, 1997). Debris flows, which tend to be

thicker, move more material, and have more variation in clast type than turbidity currents (Spence and

Tucker, 1997) and tempestites (Jones and Descroches, 1992), are postulated as the mechanism of

redeposition of the Willoughby Formation breccia. The megabreccia examined is chaotic internally which

suggests debris flows as a possible mechanism (Eberli, 1987). Debris flows are usually poorly sorted and

lack discrete bedding planes (Coniglio and Dix, 1992; Scoffin, 1987). Each of these characteristics is

evident in the breccias at “Icy Point”.

Conaghan et al. (1976) set out criteria for recognition of submarine carbonate debris flows: (1)

Coarse megabreccia is associated with thinner beds of fine breccia; the Willoughby Formation possesses

this association which is apparent in the bedding from 0.0-9.2 m above the base and the associated shift to

megaclasts in units where bedding is obscured primarily due to glacial polishing at 9.3+ m. (2) Many

deposits lack internal bedding planes, textural gradation, or other features suggestive of particle-by-particle

deposition (i.e. suspended load sediment settling out of the water column); these are characteristics of the

breccia studied as no normal or reverse grading or bedforms are apparent within the beds. (3) Several

different carbonate rock types occur and there is stratigraphic mixing of clast types; this is apparent in the

breccia studied as it is polymictic, with clasts varying from lagoonal limestone to stromatolite reef


fragments located in the same bedding planes. (4) Texture is obviously clastic and chaotic, with mixing of

clast types; the Willoughby breccia is characterized by stromatolite reef material mixed with lagoonal

limestone clasts and cementstone clasts. There is no size sorting of clasts apparent except that clast size

increases by 30 to 50 x in the upper meter of the section. (5) The large size of the megaclasts limits the

feasible mechanisms of transportation, excluding grain flows and turbidity currents; the megaclasts found

in the Willoughby breccia are of sizes (1.71 m by 1.14 m) (Table 4) large enough to exclude other

mechanisms of transportation.

Similar clast-supported, poorly sorted breccias from the Upper Precambrian-Lower Cambrian in

Spain have been characterized as resulting from debris flows (Valladares, 1995). Lock (1973) describes

Lower Cambrian – Middle Ordovician Cow Head breccias of Newfoundland as being emplaced by high

density flows. Polymictic, angular, poorly sorted clasts are characteristic of the Cow Head breccia. The

presence of the common debris flow features in the Willoughby Formation breccia indicates that it is a

product of debris flows. A similar breccia, found in the correlative Heceta Limestone Formation, was

interpreted to have been emplaced by the action of debris flows and slumping (Soja, 1990).

The multiple beds of peloidal clasts in the lower part of the stratigraphy (Figure 5) indicates that

there were repeated occurrences of debris flows. A single debris flow as the source of all of the material

would not result in the distinct bedding planes (Valladares, 1995; Coniglio and Dix, 1992; Scoffin, 1987)

that are obvious from 0.0-9.2 m in the transect. Repeated debris flows suggest that the outer lagoon and

platform margin were an episodic source of sediment transported to the upper slope environment, where

downslope gravity flows would be common (Figure 36). The elongate, angular shape of the majority of

clasts (Figure 28) suggests that they were redeposited relatively near to their source area.


Figure 36. Reconstruction of paleoenvironments believed to have existed near Drake Island, GBNP, including the presence of debris flows.

The predominance of peloid clasts and the presence of rare stromatolite clasts found low in the

section at “Icy Point” may be explained by debris flows. Debris flows initiated from the outer lagoon

would be channeled between two distinct reefs because the lower 9.1 m consisted predominantly of lagoon-

and foreslope-derived sediments (Figure 36). The spaces between the reefs would act as feeder channels

between the lagoon and the slope through which debris flows could travel, fracturing and brecciating

foreslope material as they moved (Coniglio and Dix, 1992; Scoffin, 1987). The rare stromatolite clasts

lower in the section can be explained by the channelized flows, which may have disrupted the edges of the

reefs, breaking off pieces that eventually became incorporated into the debris flow.

There is evidence that the megabreccia formed soon after early lithification of seafloor sediment.

The presence of large amounts of calcite cement (36.4% average volume in thin sections) indicate early

diagenesis in a tropical/subtropical environment (Tucker and Wright, 1990). The larger peloidal laminite

clasts contain whole, unabraded gastropod and brachiopod fossils. The presence of these whole,


undeformed, intact fossils shows that the lagoonal and foreslope sediments had solidified before they were

brecciated and redeposited.

The large stromatolite and cementstone clasts that occur upsection at the site indicate significant

collapse of the reefs. The large size and angularity of the majority of clasts suggest that they were

redeposited in relatively close proximity to their source area. Deposition of a breccia in a slope

environment (Soja, 1990) is evidenced by the characteristics of the breccia. Clasts that were rounded and

smaller would be expected if they were deposited in the deep ocean basin further from their platform

margin source (Sano and Kanmera, 1991b, 1991c). Larger blocks of material with less surrounding matrix

tend to be transported less distance than smaller fragments with more surrounding matrix due to the

available transport energy (Fuchtbauer and Richter, 1983). The lack of repeated thin beds as are seen lower

in the section suggests that the megabreccia high in the examined section was deposited by one, or possibly

a few events (Eberli, 1987). However, glacial polishing obscures evidence necessary to make a definitive

interpretation. The proportion of megaclasts to the surrounding matrix of smaller peloidal clasts also

suggests that the material was not deposited far from its source.

Sano and Kanmera (1991b) mention a number of different mechanisms that could initiate

carbonate debris flows including tsunamis, earthquakes, and other shock wave inducing events,

oversteepening of a slope due to rate of sedimentation, overloading during tidal emersion, erosion due to

wave and current action, and collapse due to uneven dis tribution of early diagenetic cements. Conaghan et

al. (1976) also mention different triggering mechanisms including earthquakes, storm waves, and

oversteepening and overloading of reefs causing collapse of slopes (Figure 37a). Mass flows such as debris

flows also tend to occur in areas that are tectonically active (Fuchtbauer and Richter, 1983). Normal

faulting as the result of extensional tectonics has been cited as a source of brecciation and redeposition

(Mustard and Donaldson, 1990).


Figure 37. Two hypotheses for the cause of brecciation in the Willoughby Formation. A. Oversteepening of a reef causing collapse and brecciation. B. Migration of faults causing progressive brecciation of foreslople, reef, and lagoonal sediments.


Sano and Kanmera (1991b) attribute the formation of Carboniferous-Permian limestone breccias

in the Akiyoshi terrane in Japan to collapse of a reef initiated by normal faulting (Figure 37b) along a

subduction trench slope during encroachment of a seamount with a trench. The Akiyoshi breccia is

characterized by poorly sorted, polymictic, angular to subangular clasts which indicate distinct source areas

and redeposition close to the source. Other deposits which Sano and Kanmera (1991b) label “broken

limestone” were recognized as well as mudstone injections; these are characteristics that are not observed

in the Drake Island breccia. The presence of an accretionary prism was recognized in Japan. The prism,

which is evidenced by basaltic rocks, overlain by oceanic sediments such as limestones and chert, indicates

a tectonically active area. The history of Akiyoshi terrane includes collision followed by structurally

stacked sequences and accretion of the seamount. The characteristics of the breccia support the theory of

tectonically induced collapse of shallow reefs.

Similar characteristics are found in the Upper Proterozoic fault-induced talus breccias of the

Olgivie Mountains, which include a predominance of angular fragments, poor sorting including fragments

from >1 cm to >100 m, and clusters of clasts of like sizes (Mustard and Donaldson, 1990). Lavoie and

Sangster (1995) mention elongate

shape and angular margins as evidence of a Lower Carboniferous tectonically induced breccia. Kale et al.

(1998) attribute the intraformational Mesoproterozoic limestone breccias of the Kaladgi Basin, with

characteristics such as subangular clasts and unsorted limestone intraclasts, to extensional tectonics of the

basin floor causing collapse and redeposition of basin sediments. Johns (1978) describes Mesozoic

carbonate breccias as being angular, poorly sorted megaclasts (1-4 m). He employs active tectonism to

explain the construction of fault scarps and steep slopes necessary for initiation of debris flows. Valladares

(1995) describes fault-induced mass flow processes resulting in breccias in the Upper Precambrian-Lower

Cambrian in Spain. Fuchtbauer and Richter (1983) connect the formation of carbonate breccias in Greece

with tectonic downwarping. A Jurassic megabreccia containing poorly sorted, angular to moderately

rounded clasts in the Tethys ocean in Switzerland was investigated by Eberli (1987). The presence of fault

escarpments and the characteristics of the megabreccia indicate that movement along faults induced mass

flows and brecciation.


A similar tectonically active environment may explain the formation of the breccia studied on

Drake Island. Evidence of a change in tectonic environment is apparent in the unconformity between the

correlative Heceta Formation and the overlying conglomeratic Karheen Formation where it is exposed in

the southern part of the Alexander terrane (Gehrels and Saleeby, 1987b). Conglomeratic strata belonging

to the Karheen Formation consist of sedimentary and volcanic rocks and overlie the Heceta Formation,

indicating a period of uplift which Gehrels et al. (1983) term the Klakas orogeny. Gehrels et al. (1983) and

Gehrels and Saleeby (1987a, 1987b) postulate that the Klakas orogeny began after middle Early Silurian

and ended by the middle Early Devonian. The event produced deformation, metamorphism, structural

uplift, topographic relief, and mountain ranges. Evidence including the thinning of strata, location of the

center of intense deformation, and location of a talus breccia suggest that the most intense area of orogenic

activity may have been in the central part of Prince of Wales Island.

The Klakas orogeny is associated with widespread faulting and compressional forces, although

subsidence and extension are thought to have followed soon after cessation of the orogeny. Ordovician-

Silurian rocks in the south and east of Prince of Wales Island (south of Glacier Bay) in the Alexander

terrane are brecciated and foliated. Gehrels and Saleeby (1987a; 1987b) attribute the deformation to

activity of the Klakas orogeny. The Willoughby breccia could represent uplift, erosion, and redeposition of

a carbonate platform due to the Klakas orogeny. Debris flows initiated by faulting and instability due to

collision (Fuchtbauer and Richter, 1983) may have caused the brecciation and redeposition of lagoon and

reef sediments to form the breccia in Glacier Bay.

Gehrels and Saleeby (1987a, 1987b) recognize six main indicators of the Klakas orogeny on

Prince of Wales Island and the structural uplift of late Ordovician-early Silurian and older strata: (1) Lower

Devonian fining-upward conglomeratic strata which contain clasts of Silurian deposits and therefore record

uplift and erosion of older deposits during deposition of the Karheen Formation. (2) upper Lower and

Upper Silurian limestone and turbidites that contain thick layers and lenses of polymictic conglomerate.

(3) Southward pinchout of Silurian strata which may indicate uplift and erosion. (4) Change from calc-

alkaline intrusive and extrusive volcanism to sodic intrusive volcanism. (5) Southwest directed movement

on thrust faults. The cause of the Klakas orogeny has not yet been determined, with both collisional activity

and intra-arc activity possibilities (Gehrels and Saleeby, 1987b). Existence of a tectonically active


environment is indicated by the polymictic, large, angular characteristics of the clasts and the significant

collapse of reefs that is suggested by the changes that occur upsection at “Icy Point”. The large extent of

the breccia, as revealed by reconnaissance work upsection on Drake Island, supports the possibility of

significant tectonic activity.

Data concerning the extent and shape of the Willoughby Formation breccia deposit, through

investigation of other breccia outcrops on the island and further reconnaissance work, would aid in

definitively determining the cause of brecciation. Correlations drawn between strata overlying the breccia

and the Karheen Formation deposits to the south would aid in limiting the possible causes of the platform

margin collapse. Other evidence suggesting the presence of an accretionary wedge or trench environment

or basin subsidence would all be helpful in determining the environment of collapse.

The recognition of the Willoughby Formation breccia as massive debris flows possibly induced

during incipient stages in the Klakas orogeny, which was first recognized in correlative rocks to the south,

can help with ongoing investigations of the Alexander terrane’s location in the Uralian seaway during the

Silurian. Further studies of the geology along the portions of Laurentia, Baltica, and Siberia that once

bordered the Uralian Seaway may reveal areas that underwent platform margin collapse similar to that

indicated by the breccias studied in Glacier Bay. The discovery of similar deposits would help to locate the

Alexander terrane as well as define the tectonic processes that led to the formation of such significant

breccias.

Summary and Conclusions Glacier Bay National Park affords interesting areas of previously unstudied geology. The

Willoughby Formation megabreccias examined on the western shore of Drake Island provide new material

to help determine the changing paleoenvironmental conditions of the Alexander terrane and its possible

location along the Uralian Seaway during the Silurian. The breccia at “Icy Point” is characterized by a

predominance of peloid-dominated clasts and a major change upsection to stromatolite and cementstone

megaclasts surrounded by small peloid-dominated clasts. Microscopic study of the breccia, including the

presence of macrofossil and microorganism assemblages unique to other areas located along the Uralian

Seaway, reveals sediment input from a lagoonal environment and the presence of discontinuous

stromatolite reefs. Characteristics of the breccia support the postulation of debris flows as the method of


deposition of the brecciated material. The presence of megaclasts upsection suggest significant collapse of

the reefs later in the history of the breccia.

There are many possible methods of initiating debris flows and collapse of the reefs, including

earthquakes, storm events, erosion, overloading and oversteepening of a reef or slope, and tectonic activity.

The Klakas orogeny, recognized by metamorphism and deformational features in the southern part of the

terrane, may have affected deposition of the Willoughby Formation to the north. The timing of tectonic

events that produced megabreccias, detrital zircons in the Karheen Formation that match ages of possible

source rocks in Baltica, paleomagnetic data coincident with Caledonide orogenesis, and the discovery of

organisms in Glacier Bay breccias that are unique to Laurentia, Baltica, and Siberia help to correlate the

Alexander terrane with other areas along the Uralian Seaway.


Acknowledgements I would like to thank everyone who was involved in the process of writing this paper. First and foremost, I would like to thank my advisor Connie Soja who put so much time and effort into guiding my field work, helping me to examine and understand the data, and critically reviewing drafts of this paper. Brian White’s help in the field was greatly appreciated. Many thanks to Stacey Joyce who assisted in field work and discussed ideas and concepts with me during our work in the field and when writing this paper. Art Goldstein, as the professor of Geology 440 Senior Research, is greatly appreciated for his reviews of drafts of the paper and for his enthusiasm and support throughout the writing process. I would also like to thank the students of Geology 440 who all read and provided critical reviews of parts of this paper. Funding for this work through Connie Soja and a grant from the National Science Foundation and from the division of Natural Science and Mathematics at Colgate University is greatly appreciated. Finally, I would like to thank my family, friends, professors, and co-leaders who listened to me talk about this project the entire semester.


Appendix 1


Appendix 1. Data collected from petrographic microscope analysis of slides.


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