Creating the Framework for a New Aquifer: The Yegua-Jackson of the Texas Gulf Coastal Plain
Paul R. Knox1, Van C. Kelley2,
and H. Scott Hamlin3
1 Baer Engineering and Environmental Services, Inc., 7756 Northcross Dr., Ste. 211, Austin, Texas 78757
2 Intera, Inc., 1812 Centre Creek Dr., Ste. 300, Austin, Texas 78754
3 Texas Water Development Board, 1700 N. Congress Ave., Austin, Texas 78711
In 2002, the Texas Water Development Board added the Eocene-aged Yegua- Jackson interval of the Texas Gulf Coastal Plain to the list of minor aquifers of Texas. The Yegua-Jackson Aquifer, however, lacks the structural and stratigraphic framework needed for groundwater availability modeling. Our analysis incorporates previous stratigraphic interpretations with 250 well logs within the outcrop and along the downdip boundaries of the aquifer. This log data has been used to subdivide the Yegua and Jackson intervals into genetic units on the basis of maximum flooding surfaces, which are presumed to be time-synchronous. The spontaneous potential (SP) and resistivity curves from 150 logs were digitized for consistent, repeatable percent-sand calculations. These new structural and lithologic data were then incorporated with trends from previous studies to produce updated genetic maps spanning the aquifer trend from Mexico to Louisiana.
The Yegua interval includes at least eight stratigraphically distinct units that have been grouped into two main layers in this study. The Jackson interval consists of at least seven genetic intervals that have also been grouped into two layers for this study. The four layers of the combined Yegua-Jackson interval are third-order units whose deposition spans 1 to 2 m.y. The 15 or more finer units which comprise these four layers are of fourth-order scale, each spanning a period of 100,000 to 400,000 yrs. In the Lower Yegua Unit, sand deposition occurs nearly equally in the Houston and Rio Grande embayments, decreasing over the San Marcos Arch. However, in the Upper Yegua and Lower and Upper Jackson units, the Rio Grande embayment appears to receive more sandy sediment than the Houston embayment.
AAPG Search and Discover Article #90069©2007 GCAGS 57th Annual Convention, Corpus Christi, Texas