ABSTRACT

The smectite-to-illite transition within Gulf Coast sediments is generally recognized as a major diagenetic process. Over the past four decades, this transition has been well characterized and has been linked to overpressure. Current models for the transition suggest dissolution of smectite and crystallization of illite, a process needing water flow to supply reactants and remove waste products. Traditionally, the transition is ascribed to burial diagenesis, whereby temperature rises with depth and compaction drives fluids into surrounding units. However, two separate age-dating studies, one based on a well in Brazoria County, TX and the other on a well in DeWitt County, TX, suggest that the transition is due to mid-Tertiary "episodic" diagenesis; i.e., occurring at a single time, geologically, in response to fluid flow through the entire system. Such a process would require rapid, pervasive flow of water through Gulf Coast sediments, an event difficult to explain in the passive-margin environment of Gulf Coast Tertiary deposition.

Recently, it has been suggested that mid-Tertiary hydrovolcanism took place beneath the Gulf Coastal plain. This interpretation is based on volcaniclastics within the Catahoula tuff (Oligocene to lower Miocene?) exposed in Live Oak County, TX. Such volcanism would result in rapid heating of subsurface water, with subsequent migration of heated fluids through the sediment column. Thus, I propose that the Gulf Coast smectite-to-illite transition may be related spatially and temporally to a mid-Tertiary volcanic event and was due to hydrothermal alteration, not burial diagenesis.