ABSTRACT

Marine limestone and shale of early Cenomanian age are exposed in complex relationships with volcanic rocks in western Uvalde County. The volcanics are largely altered to montmorillonoids, but possess relict igneous textures. They sporadically contain round boulders of limestone, and fewer boulders of basalt. Closely associated bedded clastics, consisting of granules of volcanic rock in a chalk matrix, have initial or penecontemporaneous dips up to 40°.

Limestone underlying the volcanics develops chaotic intraformational breccia, and shows crumpling antecedent to lithification. Masses of this fossiliferous limestone intrude the overlying volcanics as dikes up to 230 feet in length.

These relations are ascribed to submarine mudflows of volcanic debris, which rapidly and unequally loaded a floor of semi-consolidated calcareous ooze. Field relations suggest that the mudflows originated either by rapid erosion of a volcanic island, or by direct gurgitation of eruptive igneous material mixed with heated seawater and mud.

The occurrence sets a new date for the initiation of Mesozoic igneous activity in Texas, a cycle previously known from Turonian to Maestrichtian, and represented chiefly by ultramafic intrusives spatially associated with the Balcones fault zone.