Geologic framework, hydrostratigraphy, and ichnology of the Blanco, Payton, and Rough Hollow 7.5-minute quadrangles, Blanco, Comal, Hays, and Kendall Counties, Texas

Scientific Investigations Map 3363
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Abstract

This report presents the geologic framework, hydro­stratigraphy, and ichnology of the Trinity and Edwards Groups in the Blanco, Payton, and Rough Hollow 7.5-minute quad­rangles in Blanco, Comal, Hays, and Kendall Counties, Texas. Rocks exposed in the study area are of the Lower Cretaceous Trinity Group and lower part of the Fort Terrett Formation of the Lower Cretaceous Edwards Group. The mapped units in the study area are the Hammett Shale, Cow Creek Limestone, Hensell Sand, and Glen Rose Limestone of the Trinity Group and the lower portion of the Fort Terrett Formation of the Edwards Group. The Glen Rose Limestone is composed of the Lower and Upper Members. These Trinity Group rocks con­tain the upper and middle Trinity aquifers. The only remaining outcrops of the Edwards Group are the basal nodular member of the Fort Terrett Formation, which caps several hills in the northern portion of the study area. These rocks were deposited in an open marine to supratidal flats environment. The faulting and fracturing in the study area are part of the Balcones fault zone, an extensional system of faults that generally trends southwest to northeast in south-central Texas.

The hydrostratigraphic units of the Edwards and Trinity aquifers were mapped and described using a classification system based on fabric-selective or not-fabric-selective poros­ity types. The only hydrostratigraphic unit of the Edwards aquifer present in the study area is hydrostratigraphic unit VIII. The mapped hydrostratigraphic units of the upper Trinity aquifer are (from top to bottom) the Camp Bullis, upper evaporite, fossiliferous, and lower evaporite which are interval equivalent to the Upper Member of the Glen Rose Limestone. The middle Trinity aquifer encompasses (from top to bottom) the Lower Member of the Glen Rose Limestone, the Hensell Sand Member, and the Cow Creek Limestone Member of the Pearsall Formation. The Lower Member of the Glen Rose Limestone is subdivided into six informal hydro­stratigraphic units (from top to bottom) the Bulverde, Little Blanco, Twin Sisters, Doeppenschmidt, Rust, and Honey Creek hydrostratigraphic units.

This study used the ichnofabric index scale to interpret the amount of bioturbation in the field. Most of the geologic units in the study area are assigned to the Cruziana and Thalassinoides ichnofacies consistent with interpretations of a tidal-dominated open marine environment (sublittoral zone). Ichnofossil assemblages are dominated by Thalassinoides networks, but also contain Cruziana, Ophiomorpha, Paleo­phycus, Planolites, and Serpulid traces.

Suggested Citation

Clark, A.K., Golab, J.A., and Morris, R.R., 2016, Geologic framework, hydrostratigraphy, and ichnology of the Blanco, Payton, and Rough Hollow 7.5-minute quadrangles, Blanco, Comal, Hays, and Kendall Counties, Texas: U.S. Geological Survey Scientific Investigations Map 3363, 21 p., 1 sheet, 1:24,000, http://dx.doi.org/10.3133/sim3363.

ISSN: 2329-132X (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Geologic Framework
  • Hydrostratigraphy
  • Ichnology
  • Summary
  • References Cited

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic framework, hydrostratigraphy, and ichnology of the Blanco, Payton, and Rough Hollow 7.5-minute quadrangles, Blanco, Comal, Hays, and Kendall Counties, Texas
Series title Scientific Investigations Map
Series number 3363
DOI 10.3133/sim3363
Edition Verison 1.0: Originally posted September 13, 2016; Version 1.1: September 20, 2016
Year Published 2016
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geosciences and Environmental Change Science Center
Description Pamphlet: v, 21 p.; Sheet: 62.28 x 46.73 inches; Table; Figure; Metadata; Read Me; Spatial Data
Country United States
State Texas
County Blanco County, Comal County, Hays County, Kendall County
Online Only (Y/N) Y