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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
Colgate University Journal of the Sciences 11
Figure 1. Map of research locale in Glacier Bay National Park, Alaska. Inset map of Alaska shows location of Glacier Bay.
12 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 13
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).
14 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 15
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
16 Colgate University Journal of the Sciences
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).
Colgate University Journal of the Sciences 17
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).
18 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 19
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
20 Colgate University Journal of the Sciences
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).
Colgate University Journal of the Sciences 21
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
22 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 23
Figure 7. Stratigraphic column of rocks exposed at “Icy Point” showing upward changes of size and shape and location of collection samples.
24 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 25
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
26 Colgate University Journal of the Sciences
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).
Colgate University Journal of the Sciences 27
Figure 9. IP-1.0-99. Tracing of typical lime mudstone clasts.
28 Colgate University Journal of the Sciences
Figure 10. Tracing of large laminite clast with concentration of brachipod fossils.
Colgate University Journal of the Sciences 29
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; bound- stone; 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
30 Colgate University Journal of the Sciences
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
d cemented mud clasts
13 1.6 1.4 0.1 boundstone;
Fistulella; subround
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
d cemented mud clasts
cavities; Fistulella; 17 1 0.85 no data boundstone; cross subroun
d cemented mud clasts
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/
cemented mud clasts;
sponges; florets; no
subround
boundstone clast 17
Fistulella? 19 0.6 0.45 no data bounstone;
Fistulella subangular
cemented mud clasts;
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
Colgate University Journal of the Sciences 31
larger d concentric dones; cavity; interlaminated aragonite fans; sponge; no Fistulella? 22 8.25 5.6 no data boundstone;
sponges; subangular/
cemented mud clasts;
cavities; aragonite fans;
subround
cement fringe
Fistulella? 23 1.05 1.05 no data boundstone;
Fistulella; subround
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
32 Colgate University Journal of the Sciences
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;
Fistulella; subround
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
Colgate University Journal of the Sciences 33
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).
34 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 35
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.
36 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 37
(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-
38 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 39
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.
40 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 41
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).
42 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 43
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
44 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 45
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.
46 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 47
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
48 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 49
Figure 34. Photomicrogaph of radiating fan and block cements from sample IP-3.5A-99. Scale = 2 mm. Stratigraphic top towards top of page.
50 Colgate University Journal of the Sciences
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).
Colgate University Journal of the Sciences 51
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
52 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 53
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.
54 Colgate University Journal of the Sciences
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,
Colgate University Journal of the Sciences 55
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).
56 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 57
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.
58 Colgate University Journal of the Sciences
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
Colgate University Journal of the Sciences 59
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
60 Colgate University Journal of the Sciences
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.
Colgate University Journal of the Sciences 61
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.
62 Colgate University Journal of the Sciences
Appendix 1
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Appendix 1. Data collected from petrographic microscope analysis of slides.
64 Colgate University Journal of the Sciences
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