Kyle Barnes
15 February 2016
Structure
Group 9
Glenn Creek Quad Montana
Original deposition in the area began as flat lying layers throughout the pre-Cambrian and
Cambrian times through the Creataceous with an unconformity between Cambrian and
carboniferous sedimentary deposits. This is the result of erosional forces stemming from the
land's past existence as a plain before the Rocky Mountains formed. Some Permian and Triassic
sediments are also unpreserved. This left Jurassic and Cretaceous sedimentary units to overlie the
carboniferous and Cambrian and pre-Cambrian strata.
Unit of magma, sills, were injected into the area sometime during the Cretaceous based on the
strata lying above and below them. This is speculation based on what was provided in the map.
The sills formed from volcanic activity likely associated to the orogeny. No obvious dikes are
present to cross cut the layers and determine the age. The layer has since blended in with the flat
lying beds for the most part and does not exceed the Cretaceous layers.
Another layer of sills were also injected most likely during the late pre-Cambrian. These occur
within the pre-Cambrian layer pChc. The layer has since conformed with the other beds although
sills can be hard to predict their exact size and shape.
Folding was the first event to occur in the area. This happened after the Cretaceous sill because it
is also folded. This event preceeded the large amount of thrust faulting in the region. The land
was likely folded as a precursor to the forces of the Laramide Orogeny or part of the Sevier
Orogeny. A large, upright, north-south trending anticline is noted on the western half of the map
and visible in the cross sections. This is a close to tight fold with the axial plane of the fold
dipping almost vertically in the north and as the fold trends southward, the fold's dip rotates
toward the west. The fold becomes tighter as one progresses south. Eventually the anticline
rotated dip far enough to be inverted according to the map. This is known as the Moose anticline
in accordance with the map. The Moose anticline appears to be plunging toward the south and
have even, symmetric limbs.
A large north-south trending syncline is present on the eastern half of the quadrangle. This fold
appears to have symmetrical limbs as well. It's axial plane along it's hinge has a slight westerly
dip that remains constant. There are symbols on the map indicating it is an overturned anticline
in one small portion from the transition of the dip as it rotated in the thrusting. Some parasitic
folds can also be seen in the syncline. These “S” and “Z” shapes help to define the orientaion of
the folds and likely formed with the thrusting in the area but could have been part of an earlier
orogenic event. Other parasitic folds are also present between the major anticline and syncline.
These are hard to distinguish because of the thrusting event that proceeded the folding.
The next event to occur appears to be a series of west-dipping, imbricate, low-angle thrust faults
involving all the layers from Cretaceous down to the pre-Cambrian rocks. These thrusts are from
the Laramide Orogeny. These faults cross cut the folding present in the area thus meaning it
occurred sequentially after the folding event. The thrusting here is “thin-skinned” because there
are no crystalline rocks involved except the sill material. The bulk of the thrusting to the west is
older Cretaceous on younger Cretaceous until one gets to the southern and western parts of the
Kyle Barnes
15 February 2016
Structure
Group 9
map. A major thrust, the Renshaw thrust, brought up Jurassic rocks and helped form the
topography of Sheep Mountain. Behind this and occurring afterward, the South Fork thrust
brought pre-Cambrian rocks to the surface. Some fault fold deformation happened as the sheet
containing the pre-Cambrian and Cambrian rocks thrust onto the sheet formed by the Renshaw
thrust, which only contains Jurassic and Cretaceous rocks. This succession occurred in this order
because the layers are placed on one another.
Following the thrusting event of the Laramide Orogeny or possibly Sevier Orogeny, the area
featured some extension as can be seen in the cross sections by the Glenn fault on the south
western portion of the map. These normal faults are steep-angled and dip to the west. This
normal faulting sequence has caused a block to sink down as can be seen by relating the
relationship of the cross section. The bed pChb is a good marker to look at in the C to C’ section.
This faulting obviously happened after the thrusting as it cross cuts the thrust and the
interpretation also displaces previously thrusted units. This was part of the Basin and Range
extension event that occurred in the western North America after the Laramide Orogeny.