Abstract: Sequence Stratigraphy of Lake Basins: Unraveling the Influence of Climate and Tectonics

BOHACS, KEVIN M.
Exxon Production Research Company, Houston, Texas

The sequence-stratigraphic approach--looking at packages of rocks bounded by physical surfaces to construct a chronostratigraphic framework--works well in lacustrine strata. The expression of lacustrine depositional sequences varies widely from the "normal" marine case because lake systems themselves are greatly variable, but this does not affect the utility of the sequence-stratigraphic approach--the rocks are still deposited in layers bounded by physical surfaces that can be used as time lines. Indeed, sequence stratigraphy is especially appropriate for lakes because of its focus on integrating observations of many scales of sedimentary changes and hiatuses in a hierarchy that spans millimeters to kilometers.

Despite the great variation in modern lake systems, numerous observations of lake strata of Cambrian to Recent age reveal that only three major associations of lithologies, sedimentary structures, organic matter, geochemistry, and stratal-stacking patterns commonly recur at the meter to dekameter scale. Lacustrine strata record the integrated history of lake hydrology, which is controlled by the interaction of potential accommodation and the volume of sediment + water. The three major facies associations can be attributed to characteristic variations in these two controls at the depositional-sequence to sequence-set scale (tongue to member scale).

We propose that the three most common lacustrine facies associations correspond to distinctive lake-basin types: overfilled lake basins, balanced-fill lake basins, and underfilled lake basins, based on the interpreted relation of potential accommodation and sediment + water supply. Expression of parasequences and sequences ranges from very similar to shallow-marine sequences in some overfilled lake basins to very different in underfilled lake basins. Although named for interpreted genetic factors (to provide predictive as well as descriptive utility), each type is characterized by readily observable features such as lithofacies association and stratigraphic packaging, and possesses predictable organic geochemical characteristics (Carroll and Bohacs, 1998).

We interpret that climate and tectonics, through their strong effects on potential accommodation and sediment + water supply, exert co-equal influence on the occurrence, distribution, and character of preserved lake strata at both meso- and macroscales, mainly through their influence on the time-integrated history of lake hydrology. Unraveling the individual influences of climate or tectonics requires looking beyond stratal stacking patterns and gross lithofacies or geochemistry to the details of body- and trace-fossil assemblages, isotope profiles, fault movement, basin and spillpoint evolution, and other direct indices of atmospheric conditions or structural movement. 

AAPG Search and Discovery Article #90927@1999-2000 AAPG Distinguished Lectures