Geology of Vermont
Chazy Reef at Isle LaMotte
Condensed from text by Dr. Charlotte Mehrtens,
Vermont Geological Society Summer Field Trip, 1998
Photographs from M. Gale and J. Kim
There is a tremendous range of definitions for
what constitutes a "reef", from the liberal "anything that would break
the bottom of a boat" to those definitions which require recognition
of the ecological relationships between organisms. Following the discussion
of James and Bourque (1992) the Chazy buildups are most appropriately
termed "mounds" and not reefs due to the carbonate mud matrix surrounding
the framework organisms. The sediment between the mounds is coarser-grained,
often cross bedded packstone and grainstone. The mounds were capable
of building vertically into the surrounding water column, and thus modified
their surrounding environment. These rocks are often called "The World's
Oldest Coral Reefs" (Raymond, 1924), but could be more appropriately
termed "The World's Oldest Reefs in Which Coral Occur," but that's a
little wordy. Although coral are present, they were recent developments
in the biotic world and were not the dominant framebuilder. Bryozoa
and stromatoporoids were more successful builders of the reef, along
with stromatolites. Non-framework organisms are diverse and include
echinoderms, brachiopods, gastropods, trilobites and cephalopods; the
usual cast of characters in the Middle Ordovician seas.
The faunal changes through the Chazy reefs have been identified as representing
"succession" in the reef community over time (Pitcher, 1964). In many
reefs, the initial framework builder, for example, bryozoa, are vertically
capped by another framework builder, for example, a stromatolite. In
other words, over the lifespan of a reef horizon the environment was
best suited for the morphology and function of one group of organisms,
and as the buildup accreted into shallower water, another group of organisms
was better adapted to this new environment.
Since Pitcher's paper, paleontologists' understanding of succession
has deepened and they now differentiate between "allogenic" and "autogenic"
succession. Autogenic succession represents faunal or floral changes
which were internally driven by progressively better adapted groups
of organisms (pioneer-diversified-climax communities). Allogenic succession
is interpreted to represent faunal or floral changes in response to
external changes to the environment. In the case of a reef, sea level
change would be a prime example of an external change.
The significance of recognizing reefs or mounds of Ordovician age in
Vermont is great. First, the occurence of reefs in modern environments
at 20-30 degrees parallel to the equator provides a strong paleo-latitudinal
control for this portion of Vermont 480 million years ago. Second, the
narrow bathymetric control on modern reefs provide us with a constraint
on water depth for these rocks. Third, the detailed orientation of the
mounds (roughly aligned in a north-south belt using current geographic
coordinates) constrain the local paleogeography: the Iapetus Ocean was
to the east, Laurentia lay to the west, and this region sat on the platform
margin bordering deep water.
Contact of mound (right) and sediment (left), Day Point Formation, middle-upper Fleury Member
The mounds are circular to elliptical in shape. They can occur as isolated, mushroom-shaped buildups or as laterally-linked and coalesced mounds. The bryozoa appear to have been able to stabilize the mobile substrate and trap micrite mud within the framework, and build vertically in a series of layers. Initially the mounds accreted vertically (growth rate greater than accumulation rate) as isolates. Then, the sedimentation rate increased relative to growth rate and the Bastoma layers extended laterally and linked with an adjacent mound.
Fisk Quarry, Isla LaMotte
Fisk Quarry Walls, Isle LaMotte
|The Fisk Farm overlooks the
Fisk Quarry, now home to a variety of plant and animal life. The
quarry floor is not quite level, but dips slightly to the north,
following the incline of the bedding planes in the rock.
||The mounds or reefs of the Crown
Point Formation are of much higher faunal diversity than the underlying
Day Point buildups, with varying abundances of algae, stromatoporoids,
sponges, coral, bryozoa and stromatolites as the framebuilders.
Stromatoporoid (Pseudostylodictyon lamottensis) mounds are visible
on the quarry walls, and the mounds resemble cabbage heads. The
mounds are convex upwards, but are not symmetrical in cross-section.
By analogy to modern organisms, geologists interpret water flow
direction to be up, onto and against the steeper, buttressed sides
of the mounds. The nearly horizontal bedding planes are also visible
in the quarry walls.
Maclurites Magnus, Crown Point limestone
Cephalopod, Crown Point Formation
|The gastropod, Maclurite Magnus,
occurs in the Crown Point limestone, fine-grained, dark, silty limestone
with buff dolomitic partings. The stone was quarried for decorative
purposes. The stone from Isle LaMotte was used as building stone
for Radio City Music Hall in New York and the National Gallery of
Art in Washington, D.C.
||Cephalopods are related to gastropods.
Cephalopods lack feet and their shells are chambered. By locating
the shells of cephalopods, shaped like small wind socks, it is possible
to tell which way the water was flowing in the channels that cut
through mound horizons. The channels were conduits for water to
move from the open ocean through the reef front into the lagoon
behind. The grooves or channels are oriented perpendicular to the
oncoming wave surge.
Mound, upper Crown Point Formation
|Very little framebuilder is needed to create
a structure which can be termed a reef or mound. Most of what is
present in a reef, modern or fossil, is inter-reef rubble and pore-filling
Calcareous algae and sponges (Zittelella)
on weathered outcrop surface, Crown Point Formation
here for a geologic map of Isle LaMotte
from VGS Bulletin 9 by Robert Erwin, 1957.
James, N. and P. Bourque, 1992, Reefs and Mounds, in Walker, R. and
N. James, Eds., Facies Models, pp. 332-348, Geol. Assoc. Canada.
Pitcher, M., 1964, Evolution of Chazyan (Ordovician) reefs of eastern
United States and Canada: Bull. Canadian Petrol. Geol., v. 12, p.
Raymond, P.E., 1924, The oldest coral reef: Report of the Vermont
State Geologist, #14, p. 72-77.
For more information on modern coral reefs or fossils
the EPA Coral Reef Protection Page,
Marine biology at SUNY,
Museum of Paleontology at UC and NOAA.
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