Pennsylvania Jackfork Group Analysis
The DeGray Dam Spillway east and west walls have similar lithofacies, and are approximately 100 m apart. The authors examined 8 lithofacies types in their study area: massive sandstone, sandstone with floating mudstone clasts, pebbly sandstone, contorted sandstone, contorted shale, mudstone/shale with floating clasts, laminated and rippled sandstone and siltstone, and laminated shale. These lithofacies were interpreted based on depositional processes.
At DeGray Spillway, 794 units were measured and 189 units were measured at Kiamichi Mountain. A composite sedimentological log was created using these measurements. The samples from these locations were slabbed, polished, and examined. Thin sections were made from 3 locations of these units: top, middle, and bottom. A petrographic microscope was used to study the thin sections, based on counting 400 points per sample. Petrographic study revealed detrital matrix in thin section. Detrital matrix was determined under 3 observations, the mode of occurrence, texture, and color. In the DeGray Spillway section, there was 327 m of interbedded mudstone and sandstone, making it a noteworthy exposure to study lithofacies associations and depositional cycles in a deep-water …show more content…
The mud matrix in the massive sandstone facies ranges from 3 - 25%. The published petrographic data and the thin section study indicate the section ranges between 10 - 25% and are mostly primary matrix. If the sandstone beds were deposited from turbulent suspension or turbidity currents they would have developed normal grading and gradational upper contacts with higher amounts of matrix near the top of the bed, but gradational upper contacts are absent.
The Jackfork is composed predominately of fine-grained sandstone, and the massive sandstones are products of sandy debris flows that had enough strength to flow from the muddy matrix. The sandstone with floating mudstone facies indicates flow strength when analyzing the failure of the floating mud clasts to settle to the base.
The clast orientation and mud clasts are evidence of laminar flow in debris-flow deposits, because laminar flow conditions are present with planar clast fabric in plastic flows, rather than turbulent flow conditions in turbidity currents. Though, mudstone clasts can be interpreted as evidence that the flows were capable of erosion during travel, and debris flows were turbulent at some point during evolution. The mudstone/shale with floating clasts facies represents debris-flow deposits were cohesive, and the muddy matrix supports the