Implications for carbonate mass-wasting complexes induced by volcanism from Upper Cretaceous Austin Chalk strata in the Maverick Basin and San Marcos Arch areas of south-central Texas, USA

Abstract

The upper part of the Austin Chalk (AC-B1 subunit), as seen in a series of cores in areas of the Maverick Basin and San Marcos Arch in south-central Texas, contains a stratigraphic section of mass-wasting complexes resulting from penecontemporaneous volcanic-induced gravity-flow deposition. The types of gravity flows are variable in composition depending on sources of volcanic or carbonate sediments available and position of initiation of the flow on the volcanic mounds.The gravity flows occurred in a deeper-water drowned-shelf setting where argillaceous chalk-rich sediments were being deposited. The mafic volcanoes are part of the Balcones Igneous Province that was active during very Late Santonian to Early Campanian time. Some of the volcanoes formed mounds that created elevated areas in the photic zone or above sea level. In the shallow-water photic zones, carbonate factories formed as atolls and shoals. These carbonate factories were a source for shallow-water allochems, and along with the volcanic debris, constituted gravity flows that were likely triggered by volcanic-associated earthquakes. As gravity flows moved downslope, they eroded deeper-water, bottom sediments, which were incorporated into the flows. The two major transport mechanisms were debris flows and hyperconcentrated-density flows.The debrites appear as two types: those dominated by mud clasts and those dominated by cm-scale skeletal fragments such as inoceramid or disarticulated oyster shells. The clasts in the mud-clast-dominant debrites range in rigidity from very soft (i.e., easily deformed) to firm (i.e., resist deformation and preserved burrows). Since the mud clasts are composed of planktic foraminifers and calcispheres in a coccolith-hash matrix, they are assumed to have originated from the deeper flanks of the volcanic mounds. The rigidity of the mud clasts are likely related to initial (i.e., early) depth of burial of the unlithified mud.The skeletal-dominated debrites with disarticulated oyster shells appear to have been sourced from the upper slopes of the volcanic mounds and underwent little abrasion during debris-flow transportation. The inoceramid-fragment-dominated debrites may have been initiated lower on the volcanic mounds where these low-oxygen-tolerating bivalves would dominate the macrofauna.The hyperconcentrated-density-flow deposits are grain-rich in skeletal fragments with a mixture of shallow- and deep-water biota, suggesting some direct sourcing from shallow-water shoals and later incorporation of deeper-water sediments from the flanks of the volcanic mounds. These hyperconcentrated-density-flow deposits are positioned close to the base of the mounds.This distinct stratigraphic interval varies greatly from the rest of the Austin Chalk section in the Maverick Basin and San Marcos Arch areas. Deeper-water chalk deposition was the general background environmental setting, but it was disrupted by this period of volcanism that initiated numerous mass-wasting complexes punctuating the deeper-water chalk deposits. Within the gravity-flow deposits, volcanic components such as lapilli, ash, and glauconite-altered volcanic grains occurred along with both shallow- and deep-water carbonate allochems. Investigation of this stratigraphic unit within the Austin Chalk provides an analog for mass-wasting complexes associated with active volcanism and explains the variations in composition of the gravity-flow deposits.