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Maps showing the percentage and number of sands in several intervals of the upper Frio (Oligocene) were prepared from electric well logs to see how clearly such an analysis would reflect the depositional environments. Interpretations were based on a comparison with the sand-clay distribution occurring today in the eastern Texas Gulf Coast counties.
The position of the shore line during late Frio time was remarkably constant, as shown by the similarity in trend of the contours and in position of the maxima and minima on the sand percentage maps of all intervals. Subsidence and deposition must have been very evenly balanced throughout this time. Interfingering of sands and shales in the vertical section indicates numerous minor oscillations of sea-level which had only a transient effect on the position of the shore line.
A barrier island extended from northern Brazoria County northeastward to northern Orange County. North of the barrier island, in the barrier flat and lagoonal environments, evidence of ancient river channels is found. The Frio "hingeline," south of the barrier island, is marked by a pronounced thickening of section, particularly the shales.
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