Late Cretaceous and Tertiary Geologic History, Edwards Plateau, Llano Uplift, and Hill Country, Texas
In Central Texas, the Balcones Fault Zone separates the Gulf Coastal Plain from the elevated Hill Country/Llano Uplift/Edwards Plateau province to the west and north. The youngest geologic formations common to both provinces are the thick, widespread Edwards Limestone (Albian), and the thin overlying Georgetown, Del Rio, Buda, and Boquillas (Cenomanian) formations. Younger Cretaceous and Tertiary formations beneath the Gulf Coastal Plain have no known counterparts west and north of the Balcones Fault Zone.
The Late Cretaceous and Tertiary geologic history of the Central Texas Platform may be summarized:
(a)The vast Edwards carbonate bank was mantled beneath a covering veneer of thin (<100 feet) Cenomanian formations (Del Rio, Buda, and Boquillas [=Eagle Ford]).(b)The Central Texas Platform was covered by 500 to 1000 feet of open marine Austin Chalk (Santonian), Taylor Clay and Navarro Marl (Campanian and Maastrichtian), and Paleocene Midway Clay.(c)Throughout the Eocene (36 my), the exposed, low-lying bank (adjacent to coastal-plain and fluvial-deltaic depositional tracts) began to be gently uplifted.(d)Beginning in Oligocene time, accelerating gulfward tilting and uplift increased exposure and erosion of the buried Central Texas Platform, until Georgetown and Edwards rocks began to be exposed and eroded, their detritus deposited in alluvial aprons on the adjacent coastal plain. Balcones faulting during late Oligocene and Miocene marked the culmination of uplift along the west and north side of the Balcones Fault Zone.(e)Continued regional uplift during late Miocene and Pliocene elevated the western margins of the exposed Edwards carbonate bank, tilting the Plateau surface gently toward the southeast. Headward erosion from east and south established the basic pattern of headwater springs from the Plateau Aquifer sourcing outward-flowing streams that charged the newly created and expanding Edwards Underground Aquifer, where they crossed the faulted rocks of the Balcones Fault Zone. Approximately 5000 cubic miles of rock was eroded from the Edwards Plateau, Llano Uplift, Hill Country, and uppermost Gulf Coastal Plain as the result of Tertiary uplift and Balcones faulting.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015