Logging Requirements:
1. Find at least one concreation and measure its diameter
2. Contrast the clay pebbles with the matrix they are found in
3. Describe the effect the oxidized glauconite has had on the visible eroded part of this formation
4. What percentage of the formation shows crossbedding? In what direction?
Geology:
The Cockfield formation visible in this roadcut is one of the oldest geological deposits in Louisiana, existing since the Eocene period. It is the earliest of the Gulf of Mexico receeding and dropping sediments as it retreated to the southwest. The deposits happened against the Sabine uplift.
"Thin beds of crossbedded sandstone from the Cockfield Formation contain white pebbled weathered to clay and deep red ironstone concretions. The red iron oxide is the rusted remnants of the green mineral glauconite, which is rich in iron. Glauconite typically forms on shallow seafloorss, apparently from fecal pellets of small animals." (Spearing, 195)
Definitions:
sandstone - stone composed of sand. Sedimentary rock
siltstone - stone composed primarily of silt, much finer grained than sandstone. Sedimentary rock.
Cockfield Formation - Eocene formation that overlays the Cook Formation in this area. It is composed of brilliant red sandstone crossbedded deposits. Both are part of the Claiborne group and occur geologically in succession. However, the Cook formation is not visible at GZ but is exposed in other portions of Louisiana.
Cook Formation - Eocene formation composed of tanish-brown sandstone and siltstone.
matrix - a softer material that contains inclusions or other (usually harder) items of geologic interest. Also considered the "host" rock.
crossbedding - this is when layers of sediment (sedimentary rock) are deposited at an angle instead of in horizontal layers. based on the direction and the type of crossbedding, one can determine whether the area experienced wave or wind action during that layer's deposition.
concretion - a hard mass within a sedimentary rock that is usually much softer and more easily eroded. The concretion often occurs around a fossil,pebble, leaf, or shell.
glauconite - often called "green sand" this mineral is composed of iron potassium phyllosilicate. It is easily eroded and oxidized.
Reference:
Spearing, Darwin. Roadside Geology of Louisiana. 2nd ed. Missoula, MT: Mountain Pub., 2007. Print.
Louisiana Geological Survey. Baton Rouge: Louisiana State U, 1988. Louisiana Geological Survey. United States Geological Survey Capital Area Ground Water Conservation Commission Louisiana Department of Environmental Quality Louisiana Department of Transportation and Development. Web. 8 July 2015. .