What Happened in the Texas Hill Country after Edwards Deposition?

Peter R. Rose

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 1,500 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 5,000 cubic miles of rock was removed by erosion and solution from the Edwards Plateau, Llano Uplift, Hill Country and uppermost Gulf coastal plain as the result of Tertiary uplift and Balcones faulting.

Biography

Dr. Pete Rose (Ph. D., Geology, University of Texas, Austin) has been a professional geologist for 55 years, specializing in Petroleum Geology, E&P Risk Analysis, and Mineral Economics. Before going on his own in 1980 as an independent prospector and consultant, he worked for Shell Oil Company, the United States Geological Survey, and Energy Reserves Group, Inc, a small-cap Independent.

After 10 years as an internationally-recognized authority on economic risking of exploration drilling ventures, he founded Rose & Associates, LLP, in 1998. Pete retired in 2005; the firm continues as the global standard among consulting companies in that field, providing instruction, software and consulting services on an international scale.

Pete wrote the definitive geological monograph on the Edwards Limestone of Texas (Rose, 1972), and has continued related investigations to the present time. His 2001 book, Risk Analysis and Management of Petroleum Exploration Ventures, now in its 7th printing, is considered by many as the “Bible” on that topic, and has been translated into Chinese, Japanese, and Russian. He has authored or co-authored more than 75 published articles on an extremely wide variety of geological topics (Micropaleontology to Petroleum Economics!). He was a Fellow of the Geological Society of America, the American Association for the Advancement of Science, and Geological Society of London.

In 2005 he was the 89th President of the American Association of Petroleum Geologists, an international organization that is the largest professional geological society in the world (>37,000 members).

In 2006-07 he was a member of the National Petroleum Council, involved with their summary of the global energy situation, Facing the Hard Truths about Energy, and was also deeply involved in successful efforts to encourage the U. S. Securities and Exchange Commission to modernize its rules governing estimation and disclosure of oil and gas reserves, thus facilitating the investment component of the “shale revolution” in the U. S.

In 2013, the Geological Society of London awarded Peter R. Rose its prestigious Petroleum Group Medal for lifetime contributions to Petroleum Geology, the first American to be so recognized, and in 2014 the American Association of Petroleum Geologists honored him with its Halbouty Outstanding Leadership Award.

Pete is a 5th-generation Texan. He and his wife Alice have 5 children and 8 grandchildren, and divide their time between Austin and their El Segundo Ranch near Telegraph, Texas. In retirement, he took up a new career as a historian: in September 2012, Texas Tech University Press published his book, The Reckoning: the Triumph of Order on the Texas Outlaw Frontier, about the coming of Order and Law to the western Hill Country and Edwards Plateau regions of Texas (1873-1883). He is also well known for field trips he leads with Dr. Charles Woodruff into the Texas Hill Country that combine the topics of Geology, Wineries, and Frontier History.