NOVEMBER 2019 LUNCHEON MEETING
Chris Griffith
presents
Stratigraphy of the Upper Cretaceous Austin Chalk Group in South and Central Texas
11:30 AM Wednesday, November 20, 2019
at the Cascades
4511 Briarwood Road
Tyler, TX 75709
Cost: $25
ABSTRACT
The regional isopach map of Austin Chalk in Texas, compiled from multiple authors, shows that thicknesses do not correspond to expected basin boundaries. The isopach pattern suggests thicknesses are not only caused by structural movement, but that topographic relief existed during Austin Chalk deposition. Chalk deposition was not only a passive rain-out of pelagic sediment but was influenced by currents and regional productivity differences. Two main sequences are recognized the Austin Chalk in the outcrop and shallow subsurface in central Texas. Correlations in the south Texas area show the Maverick Basin was initially starved and was filled by the end of Austin Chalk time. A significant sequence boundary is interpreted between the two upper units, based on well log correlations showing truncation of ash beds and cores containing glauconitic beds with bored intraclasts at this boundary. This sequence boundary is interpreted to tie the main unconformity seen in outcrop in San Antonio. The Austin Chalk depositional model postulates a gradual deepening of the shelf toward the Gulf of Mexico, with a slight lip at the Edwards shelf margin and deeper water beyond. Coccolith phytoplankton were introduced in the water column as pelagic blooms and were affected by surface currents and nutrient availability. Upon settling, the sediments were affected by bottom currents and intensively burrowed. Updip, the sediment is light colored, massive, with diverse burrows and mollusks in biostromes. Downdip, the sediment contains more inoceramids, less diverse burrowers, more clay and organic matter, and laminated intervals, indicating periods of dysoxia. Erosion occurred during sea level fall. The sediment was reworked, and glauconite was incorporated into the sediment, during transgression.
BIOGRAPHY
Christine Griffith is a retired petroleum geologist and a Ph.D. student in the Department of Geology and Geophysics at Texas A&M University. She is studying the regional sequence stratigraphy, ages, and rock properties of the Austin Chalk in Texas. Christine received her B.S. in Geology at the University of Illinois-Champaign and a M.S. in Geology at the University of Wisconsin-Madison. She had a long career at Shell Oil as a geologist and subsurface coordinator working with geoscientists and engineers in exploration and development projects in the onshore United States, Alaska, and deep-water west Africa and Brazil.