GEOLOGY AND GRAVIMETRY OF THE MUSTANG HILL LACCOLITH, UVALDE COUNTY, TEXAS
Mustang Hill is one of about 90 small intrusions in Cretaceous sediments paralleling the Balcones fault zone of central Texas. Igneous activity in the province extends from earliest Cenomanian to Maestrichtian and includes some extrusive rocks. Faulting probably overlaps this period but is mainly later. The Mustang Hill intrusion, 1 ½ miles in diameter, is an analcitic basalt with 27 percent olivine, resembling mafic members of alkaline basalt suites from Scotland and Australia. Presence of distinctively diopsidic clinopyroxene, with absence of orthopyroxene and pigeonite, is confirmed as characteristic of this nonorogenic magma type. Limestone flags (Eagle Ford formation) in the concordant floor of the intrusion dip 3°–10° centripetally and have been locally crumpled by the basalt in small-scale decollements over more massive limestone. Broader folding is related to subsidence beneath and around the intrusion. Field relations and geophysical findings justify the name laccolith for this quasi-concordant mass. A detailed gravity survey reveals an asymmetrical maximum over the intrusion with 2.2 milligals of Bouguer anomaly relief. The highest point on the anomaly is in the southwest portion of the exposed igneous rock, whence the anomaly drops off sharply to the southwest, but has a relatively gentle gradient change to the northeast. The general shape of the anomaly suggests that the high point is associated with an igneous feeder, and the gentle gradient to the northeast is probably associated with a tabular body tapering away from the feeder.