About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 725

Last Page: 725

Title: Sedimentation Rates and Illite-Smectite Diagenesis: ABSTRACT

Author(s): James J. Howard

Article Type: Meeting abstract

Abstract:

The percentage of expansible smectite layers in a mixed-layer illite-smectite (I/S) clay has been correlated often with temperatures in a sedimentary basin. Sequences in Tertiary Gulf Coast sediments demonstrate that sediment accumulation rates exert a great influence upon I/S expansibilities by influencing thermal history. In sediment piles with high accumulation rates, the greater redistribution of energy produced by increased forced migration of fluids results in lower temperatures and thermal gradients and, as a consequence, the I/S clays are expected to react toward illite at slower rates. I/S clays from deltaic Wilcox (Eocene) shales have 50% expansible layers, in contrast to 25% expansible layers in I/S clays in marginal shelf Wilcox sediments found at similar pres nt-day burial depths and temperatures. The more expansible I/S assemblages in the rapidly accumulating deltaic sediments are consistent with the predictions of energy transport theory in compacting sediments. As these Eocene I/S clays are now being buried under roughly equal thicknesses of sediment at similar rates, the difference in thermal gradient imposed by the original deltaic and shelf margin Wilcox sedimentation is being slowly reduced until both paleogeographic areas have roughly equal geothermal gradients. At similar present-day temperatures around 100°C, the important consideration is that the I/S clays in the slowly deposited shelf margin sediments reached the minimum reaction temperature before the clays deposited at the same time in deltaic sediments. Implication for I/ diagenesis is that the maximum temperature encountered is not as important as the rate at which minimum reaction activation temperatures in the basin are achieved, and that use of expansibility changes in I/S clays as a geothermometer needs to be reevaluated.

End_of_Article - Last_Page 725------------

Copyright 1997 American Association of Petroleum Geologists