Joe Bartlett
10/1/02
Geology 101
Report on the Geologic Evolution of Lessor’s Quarry, South Hero, VT
Abstract
The purpose of this report is to create a complete geologic sequence of the events
leading to the formation of Lessor’s Quarry as observed through several days of field
collection and interpretation. The Quarry is located in South Hero, VT. This report will
establish the purpose and significance of the study and interpret the data. This report will
document the rare opportunity to view a geologic history in three dimensions present at
Lessor’s Quarry. The stratigraphic composition of the limestone was recorded through a
detailed study of the bedding planes. The shape, size, and orientation of all walls in the
Quarry were documented in a detailed base-map. Using the structures and bedding on
the East Wall a composite cross-section was constructed to relate the features on the
North and South Walls. Using trigonometry, the exactly location of the walls relative to
each other was established to allow for the formation of theories linking the geologic
history of the Quarry. Assumptions taken from the Fault-Bend Fold model were applied
to the structures in the Quarry to successfully interpret the observations made in the field.
Measurements and observations were taken to further explain smaller scale features
throughout the Quarry. From all of this data, a complete sequential geologic history was
compiled for the Quarry. This sequence is aimed to explain all of the major features:
bedding planes, cleavage, faults, folds, and veins observed in the Quarry. With the aid of
figures, this report will explain in detail the geologic sequence of Lessor’s Quarry.
Introduction
The purpose of this report is to produce a sequential model of the events which
formed the geologic structures observed at Lessor’s Quarry. This will be accomplished
through relating the structures present on the North and South Walls using the connection
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of the East Wall to establish the geologic sequence. The three walls present in the Quarry
represent an excellent study area because of the three-dimensional relationships which
can be produced.
In order to create the three dimension geologic sequence of the Quarry, many
different types of observations and calculations were needed.
A location map was
created to display the location of the Quarry (Fig 1.) A detailed study of bedding
structures and composition were used to determine the types of limestone present in the
quarry (Fig 2). A detailed base map was constructed to document the shape, azimuth and
three-dimensional orientation of the bedding and structures present (Fig 3). Using the
geometry of the East Wall, a cross-section was produced to relate the structures on the
North and South Wall (Fig 4). Numerous other observations were recorded to document
the features of cleavage, faults, and folds throughout the Quarry (Fig 5-7). From these
observations a final model geologic sequence was created to explain all of the features in
the Quarry (Fig 8).
Results
Stratigraphic Data
The stratigraphy of the Quarry was studied in detail in an approximately 1 meter
by 20 cm area in the North corner of the quarry located just below the Stanley fault. The
study area contains multiple distinct bedding layers with distinguishable variations in
grain size, color, and fossil content (Fig 2). The stratigraphy is dominated by alternating
layers of mudstone and bioclastic limestone containing bryzoans and crinoid fossils. The
fossils are found in varying conditions ranging from well-preserved to a very dense fossil
hash of broken and compacted fossils. The grain sizes vary from approximately fine to
coarse silt. Most of the stratigraphic column consists of irregularly alternating beds of
mudstone and bioclastic limestone with two distinct layers of fossil hash. All of the
bedding layers have distinct lower boundaries but several layers gradually change from
coarser to finer sediments from bottom to top.
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Structure of the Quarry
The Quarry consists of 15 walls of varying lengths with one inferred wall to
represent the opening of the Quarry. The structures of interest in the Quarry are found
throughout what will hereafter be described in terms of North, East, and South Walls.
The structure of the Quarry can be simplified into these three main wall sections (Fig 3).
The geologic features found in the Quarry are dominated by several large faults and folds
(Fig 4). The North Wall contains evidence of two folds and three faults. The North Wall
is split horizontally by the Stanley Fault. The North Wall Fault and the Synclinal Fault
are also found on the North Wall above and below the Stanley Fault respectively. The
Synclinal Fault splays into many smaller faults on the western side. The South Wall
contains the Lessor’s Quarry Fault and the South Wall Fault. The Lessor’s Quarry Fault
spans the width of the South Wall and the South Wall Fault is heavily folded. A small
cluster of minor faults is also visible on the South Wall.
Fault-Fold Relationships
Faults and folds are the most prominent features of the Quarry. Several of the
faults are incorporated into the folded structures on both the North and South Wall. The
Synclinal Fault is heavily folded similar to the bedding layers both above and below the
Fault. The North Wall Fault is also folded sharply in the same pattern as the surrounding
bedding layers. The fold of South Wall Fault appears to be very similar to that of the
North Wall Fault (Fig 4). A minor fold is visible on the South Wall with apparently no
faults running through it.
Cleavage and Veins
Cleavage is found throughout much of the Quarry on both the North and South
Walls (Fig 4). The cleavage consists of long narrow gaps in the parent material which
contain a very fine silt/clay arranged in high angle beds. Some of the cleavage visibly
cuts through structures such as fossils (Fig 8 B). The cleavage occurs somewhat evenly
throughout the Quarry. Fault-zone cleavage is found along both the Stanley and Lessor’s
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Quarry Faults.
Other than the cleavage found in folded areas which shows some
evidence of deformation, most are dipping approximately 50-70 degrees to the East. The
cleavage zones near faults also show some evidence of deformation.
Most of the
observed veins occur near to zones of faulting. Several large calcite deposits are visible
along the Stanley and Synclinal Faults. A high concentration of small veins of calcite is
visible on the western side of the Synclinal Fault. Some of the veins and calcite deposits
along the Stanley Fault show Slickenlines with an azimuth of approximately 318 degrees.
Cross-cutting Relationships
Many cross-cutting relationships are visible in the major structures present in the
Quarry including truncations and discontinuities, overprinting relationships, and fold
relationships (Fig 4). On the North Wall, the Stanley Fault truncates all of the other
major structures including the North Wall Fault, Synclinal Fault and fold, and the fold
above the Synclinal Fault. A discontinuous bedding layer is also visible along the
Synclinal Fault where a distinct layer of fossil hash is offset across one of the smaller
faults on the Western side of the Synclinal Fault. The minor fault cluster on the South
Wall also shows evidence of discontinuity. All of the faults in the cluster are truncated
by the vertical minor fault, which is then truncated by the Lessor’s Quarry Fault.
Overprinting relationships are visible on both the North and South Wall along the Stanley
and Lessor’s Quarry Faults (Fig 5). The Quarry contains evidence of several crosscutting fold relationships. All of the folded structures on the North Wall appear to be
cross-cut by the Stanley Fault.
With enough distance above or below the folded
structures level out and continue at a much lower angle. The same statement is true for
the South Wall where the Lessor’s Quarry Fault cross-cuts a zone of folding.
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Discussion
Environment of Deposition
The data obtained from the stratigraphic column allows for the formation of
several theories concerning the environment of deposition present at what is now
Lessor’s Quarry (Fig 2). The bedding layers suggest that the depositional environment is
similar to that of the Continental slope where normal deposition is occasionally
punctuated by high-energy events such as storms.
The silt sized material and the
presence of marine fossils suggest that the deposition took place in a low energy marine
environment. The lack of ripples and other similar structures in the bedding show that no
strong currents such as tides or wind were present during deposition. The bryzoans and
crinoids both come from the Neritic zone, where wave action would have influenced
deposition. High-energy events such as storms would have carried these organisms out
of the Neritic zone and deposited them on the Continental slope. These high-energy
events are represented by the fossil hash found in the column. The high-energy event
theory is also supported by the evidence of settling in several of the bedding layers. If
large amounts of sediments were blown out to sea they would gradually settle with the
larger particles on the bottom, gradually fining upwards.
Structural History
Origin of Cleavage
Spaced cleavage forms in limestone when compressional forces squeeze water out
of the rock and cause the dissolution and subsequent remove of material from the parent
rock. The cleavage forms at a plane perpendicular to the pressure acting upon the rock.
The high angle spaced cleavage which dominated Lessor’s Quarry shows that the
pressure applied to the rock was horizontal. The formation of cleavage was the first
major geologic event to influence the material in the Quarry after deposition. The
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formation of cleavage was a continuous process throughout the other events of the
geologic sequence. This continuous formation suggests that the compressional forces on
the limestone were fairly constant due to the majority of the cleavage forming in similar
patterns and at similar dips. A second form of cleavage: fault zone cleavage, is also
evident in the Quarry. This type of cleavage forms along faults as a result of the highly
pressurized water flowing through the faults during movement.
Cleavage, Vein, and Fault Relationships
Cleavage and veins both occur most strongly in zones of faulting. The calcite
present in the veins suggests that the dissolved material taken from the cleavage was
precipitated out of the pressure solution in the veins. This implies that the pressure in the
veins was less than that of the cleavage. The large blocks of calcite along faults in both
the North and South Wall show that the pressure solution also flowed through the faults
and possibly helped with their movement as shown with the Slickenlines.
The
Slickenlines on the North Wall show that the Stanley Fault was moving at approximately
318 degrees. Deformation of the cleavage along faults also helps to determine the
direction of movement. Deformed cleavage along the Stanley Fault, Synclinal Fault, and
Lessor’s Quarry Fault showed evidence of movement. The direction of movement along
a fault can be related to the concavity of the cleavage (Fig 5). For both the Stanley and
Lessor’s Quarry Fault the deformed cleavage show that the top of the fault was moving to
the west with respect to the bottom. The fault zone cleavage forms perpendicular to the
pressure in the fault and has a similar dip to that of the deformed cleavage.
The cleavage along the Synclinal Fault is crucial to determining the geologic
history of the formation. Deformed cleavage, fault zone cleavage, and veins are all
evident along this fault.
Deformed cleavage and fault zone cleavage are observed
suggesting movement in both directions on the upper part of the fault. From this a rough
sequence of movement and cleavage formation along the Synclinal Fault can be deduced
(Fig 7). Cleavage formed before the initial movement of the fault would have been
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deformed with the formation of the Synclinal Fault. During the Fault-Bend Fold process
additional stress was placed upon the Fault and it moved again only in the opposite
direction deforming more cleavage which had formed after the initial movement. Finally,
a single undeformed cleavage formed in the middle of the fault after all of the movement
was finished.
Fault-Fold Evolution
The theories behind the Fault-Fold Evolution of the Quarry are mostly based on
the connections made between the North and South Walls using the East Wall as a
reference (Fig 4). By connecting the observed structures it is possible to both determine
the relative age of the ends of the Quarry and to create a rough sequence of the Fault-Fold
events. The structures used to connect the two walls involved the Synclinal Fault and the
folded bedding planes present above the South Wall Fault. This was accomplished by
measuring the approximate dip of the quarry and by tracing a feature from the top of the
South Wall to where it hit the floor of the Quarry and then measuring the distance from
that point to the North Wall. A trigonometric relationship was then established to
determine how far below the bottom of the North Wall corresponded to the top of the
South Wall.
It is assumed that faults form along a flat plain and can then be folded and
deformed by other forces. Coupling this assumption with the Fault-Bend Fold model it is
evident that two such events occurred in the Quarry. The folding and truncation of the
North Wall Fault and the South Wall fault into the Stanley and Lessor’s Quarry Fault
respectively and the folding of bedding all suggest connections to the Fault-Bend Fold
model (Figure 8C+D). If ramps are projected to the east of the Quarry the model is
strongly supported by the observations taken in the Quarry. Using the model, the Stanley
Fault and the Lessor’s Quarry Fault would each serve as upper flat faults for Fault-Bend
Fold events. The model also explains the formation and shape of the Synclinal Fault and
the subsequent connection of the North and South Wall.
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Geologic Sequence
The first event in the geologic evolution of Lessor’s Quarry was the deposition of
the bedding planes. These were flat and undeformed during deposition (Fig 8A). The
bedding planes are cross-cut by the cleavage suggesting that the cleavage formed next.
This cross-cutting is found in the numerous overprinting relationships in the Quarry (Figs
5,6, + 8 B1). This implies that after deposition, horizontal compressional forces began to
act upon the rock. These compressional forces cause water to be squeezed out of the
limestone and under pressure this water dissolves calcite out of the rocks (Fig 8 B). The
next event was the formation of the minor fault cluster. The minor fault cluster also
formed in a sequence with the horizontal faults forming before the vertical fault which
truncates them. The Synclinal Fault and the South Wall Fault also formed around this
time. The exact order in which these faults formed is not able to be determined but
through cross cutting it is known that the minor fault cluster formed before the South
Wall Fault, which truncates the vertical fault in the minor fault cluster (Fig 8 C).
The formation of the Lessor’s Quarry Fault and the development of the first FaultBend Fold was the next major event (Fig 8 D). The ramp was to the East of the Quarry
so all the material pushed in by this event came from the East. The South Wall Fault was
truncated and uplifted by the ramp and folded into the Lessor’s Quarry Fault. The
folding was continued upwards and is what deformed the Synclinal Fault. Because the
Stanley Fault truncates the Synclinal Fault it is known that the first Fault-Bend Fold
happened before the formation of the Stanley Fault. The Stanley Fault was the upper flat
fault of the second Fault-Bend Fold event in the Quarry (Fig 8 E). Similar to the first
event, the ramp was formed to the East and material from the East was pushed in. The
North Wall Fault was truncated by the ramp and folded. The bedding layers below the
North Wall Fault show evidence of folding and truncation by the Stanley Fault identical
to that present on the South Wall. The last major event to affect Lessor’s Quarry was the
regional uplift which resulted in the tilting of all structures approximately 15 degrees to
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the North (Fig 8 F). Because all of the structures had a similar dip angle it is known that
the tilting of the material had to have occurred at the end of the sequence. The uplift
could have been caused by any number of geologic events. In order to determine the
exact cause of the uplift a much more detailed study would be required.
Conclusions

The limestone found at Lessor’s Quarry was deposited in a low-energy deepwater marine environment which was punctuated by brief high-energy events.

This deep-water low-energy environment was found on the Continental Slope.

The materials and structures present at Lessor’s Quarry allow for the threedimensional modeling of the geologic history of the area.

The information present at the Quarry allows for the connection of the events
recorded on both the North and South Walls using the East Wall for reference.

The sequence of geologic events was compiled from observations of bedding
planes, cleavage, veins, faulting, and folding.

Using a composite cross-sectional diagram of the Quarry the structures were
related to the Fault-Bend Fold model.

The Synclinal Fault formed through a complicated series of processes (Fig 7).

The geologic sequence of Lessor’s Quarry was determined to be as follows:
o Bedding planes are laid down in a depositional environment similar to that
of the Continental Slope
o Horizontal compression is applied to the limestone causing the formation
of cleavage and veins which continue to form through the duration of the
sequence
o Continued compressional force causes faulting in a sequence determined
by cross-cutting relationships
o Two Fault-Bend Fold events occur causing deformation of a majority of
the Quarry
o Regional uplift tilts all structures approximately 15 degrees to the North
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