Isochronous Correlation of Sandstone Facies Within and Between Growth-Faulted Frio Intra-Slope Subbasins: Common Correlation Pitfalls

L. Frank Brown Jr., Robert G. Loucks, Ramón H. Treviño, Ursula Hammes, Patricia Montoya, and Randy L. Remington
Bureau of Economic Geology, The University of Texas, Austin, Texas

Lithofacies, e.g., lowstand sandstones, must be correlated on the basis of similar log shape. Such pattern correlations equate only lithogenetically similar facies, not chronostratigraphically equivalent facies. Consequently, interpreters must confirm that log-facies maps are based on time-equivalent facies not genetically similar log patterns.

On-shelf highstand and transgressive sandstone facies do not correlate with off-shelf lowstand sandstone facies within the same subbasin simply because they occur between the same marine-condensed sections, i.e. same biozone. On-shelf and off-shelf sandstone depositional episodes are temporally unique. Hence, the facies are diachronous. Similarly, assumptions that in different subbasins, lowstand sandstone facies that occur between the same condensed sections (i.e., microfossil markers) are chronostratigraphically equivalent may lead to serious miscorrelations. Our research indicates that subbasin lowstand systems tracts become younger basinward as depositional dip, even if they occur between the same condensed sections. However, subbasins on the downthrown side of the same regional growth faults system (depositional strike) were filled contemporaneously, but one must be certain that inferred, time-correlatable, lowstand sandstone facies in different subbasins are components of the same 3rd-or higher-order sequence before accepting time equivalence. Verifying that the marine condensed sections at the top of the transgressive tracts within each sequence are faunally equivalent ensures equivalency.

Sandstone facies within high continuity on-shelf highstand and transgressive systems tracts can be lithostratigraphically correlated with confidence across large areas using the condensed section “envelope” as proof of time-equivalence. However, such on-shelf progradational or retrogradational lithostratigraphic units are typically cut by high-frequency time-lines, precluding absolute equivalency throughout the units.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004