Abstract

A volcanic sediment apron in the Tascotal Formation consists mostly of sand-sized debris derived from nearby volcanic centers. In the apron, a progradational sequence of sheetflood deposits gives way upward to aggradational deposits formed as major feeder channels were gradually abandoned. Sheetflood deposits, here termed thin fining-upward sequences, begin with a lens of structureless or trough cross-bedded medium sand to granular coarse sand, which overlies a scour surface. The basal lens grades up into thin-bedded or structureless fine sand. Bedding planes in the upper part of the sequence are marked by evidence of episodic deposition such as sun-cracked mud drapes or burrowed, root marked or tracked horizons. Aggradational channel deposits, or thick fining-upward sequences, begin with a thin--one or two clast thick--cobble or boulder conglomerate overlain by fining-upward subsequences that include tabular crossbeds, horizontal lamination, low-angle trough cross-bedded sandstone and mud-rich units. Volcanic activity contemporaneous with formation of the Tascotal apron was characteristically explosive, producing abundant pyroclastic material in addition to lava flows. A cover of fine debris sufficient to prevent erosion of lavas and ash flow tuffs that would contribute coarser material to the apron was maintained over the volcano slopes and adjacent uplands. With cessation of the activity of source volcanos, erosion quickly stripped this fine cover, and began to erode the flow rocks. With this, gravel replaced sand as the predominant sediment material. This change of grain size can be attributed to the cessation of volcanic activity rather than tectonic movements or climatic change.

First Page Preview

First page PDF preview
You do not currently have access to this article.