Regional gravity surveys have been conducted (1) between Bryan-College Station and Austin, Texas, across the margin of the Gulf Coast geosyncline and the buried Ouachita fold belt, and (2) across the northwestern Llano uplift, in the Llano-Mason-San Saba-Brady area. These new gravity data have been incorporated with those previously published in order to construct a regional gravity-anomaly map, contoured at a 5-mgal interval, of a large part of central and southeast Texas.

Gravity anomalies in the Llano region can be interpreted readily in terms of exposed major Precambrian rock units. Characteristic anomaly patterns then can be used to interpret basement lithology where concealed around the periphery of the uplift.

Gravitational effects of the thick wedge of Cenozoic and Mesozoic sedimentary rocks in the Gulf Coast geosyncline were calculated for several different density contrasts; thicknesses were based on extrapolations of regional well data. Similarly, gravitational effects of Paleozoic clastic rocks in the "foreland basin" between the metamorphosed Ouachita facies and the crystalline rocks of the Llano uplift were calculated

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for several density contrasts and basin configurations.

Combined gravitational effects of the models have been applied as corrections to the Bouguer anomaly map to obtain a gravity-anomaly profile that generally represents effects of changes in crustal thickness or density. The granitic crust is considerably thicker under the Llano uplift than under the Gulf Coast geosyncline.

At present, the region is at or near isostatic equilibrium, but near the end of the Paleozoic it was out of equilibrium, with an excess mass at the present site of the Gulf Coast geosyncline. The writers speculate that gulfward migration of depocenters during Mesozoic and Cenozoic has taken place in response to a mechanism for gradual restoration of regional isostatic equilibrium.

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