119-5 Mapping the geology, structure, hydrostratigraphy, and potential habitat of Streptanthus bracteatus (bracted twistflower )

Session: Recent Investigations of the Hydrogeology Edwards (Balcones Fault Zone) Aquifer, the Trinity (Hill Country) Aquifer, and Their Interactions, South-Central Texas

Presenting Author:

Authors:

Abstract:

Streptanthus bracteatus (bracted twistflower) is a geo-endemic annual plant found only along the Balcones Escarpment of the Edwards Plateau of central Texas. Previous studies reported a close association of documented populations of S. bracteatus with the contact between the Edwards Group and Glen Rose Limestone. Overlaying precise geographic coordinates of S. bracteatus individuals with recently published hydrostratigraphic maps of Bexar, Medina, and Comal Counties, Texas, revealed that most S. bracteatus individuals occurred within the basal portion of the  of the Edwards Group, which immediately overlies the upper Glen Rose Limestone. However, there has not been any revision of the existing hydrostratigraphy within Travis County, Texas, where 41 percent of the species’ known populations occur. On-site observations were made to map the geology, structure, and hydrostratigraphy of the documented S. bracteatus populations at the Barton Creek, Mt. Bonnell, Bull Creek, Gus Fruh, and Ullrich units of the Balcones Canyonlands Preserve in Travis County. Based on the combined data we determined that S. bracteatus habitats tend to correlate with more transmissive hydrostratigraphic units in the lower part of the Edwards and upper zone of the Trinity aquifers as well as areas containing faulting (fracturing). We hypothesize that an essential feature of these occupied habitats of S. bracteatus is greater availability of moisture, which is driven by the effects of hydrostratigraphy and faulting.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7525

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