Geologic History Narrative
Brooke Miskowski
When ‘Rock Unit A’ was deposited (Mica Sandstone, Shale, and Fossiliferrous
Limestone,) the environment where it is located was undergoing a marine transgression;
causing sea level of the area to rise. The way the sediments were stacked horizontally originally
showed a fining upward pattern as a sandstone layer was laid down, followed by a shale layer,
and lastly a limestone layer. The first event to happen in this area was the Mica Sandstone
Formation being deposited horizontally. In looking at this rock formation, there are
symmetrical ripples contained throughout it, informing us of the nature of the sea flow at the
time of deposition. Because the ripples are symmetrical in structure, the flow and current of
the sea at this time was bidirectional, depositing this specific rock formation in a relatively calm,
yet slightly oscillating sea environment. In this rock formation, the presence of crinoids and
brachiopods can be found in some samples, giving rise to the relative age date of this rock
formation ~ Early Paleozoic.
After the Mica Sandstone rock formation was deposited horizontally, the Sedimentary
Shale Formation was deposited on top of it horizontally as well. The Sedimentary Shale
Formation contains trilobites, giving us another clue as to the relative time this formation was
deposited. Next, on top of the Sedimentary Shale Formation, a layer of Fossiliferrous
Limestone was deposited horizontally. Rugose Coral fossils can be found in various samples of
this rock formation. In looking at the succession of the beds, we know that the oldest layer is
on the bottom of the horizontal stack, which in this case is the Mica Sandstone Formation; the
youngest being the Fossiliferrous Limestone. In looking at the rocks and fossils alone we are
still unsure if our conclusion about where the oldest rocks are located is correct.
After the deposition of ‘Rock Unit A,’ a ductile deformational event took place, causing
the beds to fold. The forces acting on the unit were compressional forces which was what
caused the entire rock unit to fold. In looking at the strike and dip of the area, we see that the
strikes on both sides of the fold are striking north. In the West, there is an approximate dip of
45 degrees in the Eastern direction. In the East, there is approximately a 30 degree dip in the
Western direction. From the way that the beds are dipping, and the angles that they are
dipping at, we can conclude that this folding event caused the creating of a non-plunging,
asymmetrical syncline. Now recognizing this formation, we can conclude that the youngest
rocks are in fact at the center of the formation (Fossiliferrous Limestone) and the oldest rocks,
(Mica Sandstone,) are at the outsides of the fold.
Many years passed after the deposition and folding of ‘Rock Unit A,” after the Early
Paleozoic, before ‘Rock Unit B,’ (Quartz Sandstone Formation, Glauchonite Formation, and the
Iron Rich Sandstone Formation) came to be deposited. In looking at the non-recorded time, we
can tell that there is an unconformity present between ‘Rock Unit A’ and ‘Rock Unit B,’ more
specifically an angular unconformity is created underneath the newly deposited rocks. In
analyzing the fossil types in ‘Rock Unit B,’ (Echinoderms, clams, the presence of fish, and bone
fragments,) we can conclude that this unit was deposited sometime during the Mesozoic –
Cenozoic time period.
The first thing that happened in this rock unit is that that some unidirectional body of
water entered the area, depositing the Quartz Sandstone Formation horizontally. We know
that this formation was deposited in a unidirectional flow environment due to the presence of
asymmetric ripples in this specific rock type. The ripples contained some tuning fork junctions
(transitioning from 2D to 3D ripples,) suggesting that the flow speed of the water in the area at
the time of deposition was moderate to fast (transitional) as the 2D crests were becoming for
3D structurally. Echinoderms and Clams can also be found in the Quartz Sandstone Formation,
again suggesting that this rock formation was laid down sometime during the Mesozoic –
Cenozoic.
After the deposition of the Quartz sandstone, some body of water deposited the
Glauchonite Rock Formation Horizontally. The presence of fish poo in this rock type tells us
that fish were present during the time of the deposition. After this body of water deposited the
Glauchonite Formation, another Formation (Iron Rich Sandstone,) was deposited on top of it
horizontally. Vertebrae fossils were found in this rock formation, suggesting the presence of
larger animals. In analyzing the fossil progression, from echinoderms and clams, to fish and
eventually Vertebrae we can conclude that the Quartz Sandstone Formation is the oldest of
‘Rock Unit B,’ followed by the Glauchonite Formation, with the youngest being the Iron Rich
Sandstone. As the animals and organisms present in the area were evolving, the different rock
unit composition changed over time as well due to the fact that the animals were changing the
environment they lived in. (For example, the sandstone over time started to contain fish poo,
and eventually an iron impurity suggesting the abundance of oxygen.)
After these rock formations were laid down horizontally, an event causing brittle
deformation occurred in the area. An extensional force was pulling the area apart, resulting in
a normal faulting of ‘Rock Unit B.” In this area we know it was a normal fault because the
hanging wall moved down with respect to the footwall. The rocks originally were stacked
horizontally with the Iron Rich Sandstone Formation on the bottom, followed by the
Glauchonite Formation on top of that, and the youngest on top; the Quartz Sandstone. When
the fault happened, the other rock unit was moved downward. We can tell this because of the
way the beds show up on the formation after they are eroded to the present day surface; which
is the last event to happen.