2024 GCAGS Journal, Vol. 13
TITLE
Estimating Water Depths of Upper Cretaceous Pilot Knob Volcanic-Related Strata: The McKown and Pflugerville Formations and Pyroclastic Ash at the Lower Falls Section, McKinney Falls State Park, Austin, Texas
AUTHOR(S)
Robert G. Loucks, Robert M. Reed, and Priyanka Periwal
ABSTRACT
The McKown and Pflugerville formations and pyroclastic ash beds at McKinney Falls State Park in Austin, Texas, are closely associated with the Lower Campanian Pilot Knob volcano. This volcano is one of over 400 known Late Cretaceous volcanoes in the Balcones Igneous Province. The center of the Pilot Knob volcano is 1.5 mi (2.4 km) southeast of the McKinney Falls State Park. The McKown strata at the Lower McKinney Falls within the park have been interpreted as a beach complex deposited on the shallow-water north flank of the Pilot Knob volcano. Previous researchers based this interpretation on the presence of a shallow-water macrobiota and the coarse-grain texture of the limestones. However, a new investigation of the Lower McKinney Falls outcrops has developed evidence that the McKown limestones were deposited in deeper water (150 to 300 ft [50 to 100 m]) on the flank of the Pilot Knob volcano by gravity-flow processes that formed carbonate debrites and hyperconcentrated-density-flow deposits. The new evidence comes from micropetrography data that demonstrates the matrix of the packstones is dominated by coccolith hash and that there are calcispheres and planktic foraminifers in both the packstones and grainstones. The composition of the McKown limestone is a mixture of shallow- and deep-water biotas, indicating the shallow-water biota were resedimented into a deeper-water setting. Also, McKown bedding relationships with the pyroclastic ash beds below indicates deposition in a low-energy setting.
The pyroclastic volcanic beds contain coccoliths and are interpreted as being deposited in a deeper-water, lower-energy setting. Also, the Pflugerville limestone section is very similar to the McKown limestone section and evidence indicates Pflugerville limestones were not deposited in a shallow-water setting. The evidence produced by the present investigation substantiates that the carbonate strata at the Lower McKinney Falls were not deposited in a beach or shoaling complex but were deposited on the deeper flank of the Pilot Knob volcano. The concepts developed by this investigation of these deeper-water carbonates can be applied to interpreting the depositional setting of other carbonates associated with the Balcones Igneous Province volcanic mounds.
PAGE(S)
53-74
DOI
https://doi.org/10.62371/DSUE4387
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