Chronostratigraphy, depositional rates, continental margin progradation, and growth-fault dynamics within the tertiary wedge, San Marcos arch, northwest Gulf of Mexico

Sedimentation, growth fault slip, and shelf-margin progradation rates were determined for a transect of Tertiary sediments along the axis of the San Marcos arch in the northwestern Gulf of Mexico basin. This low relief area was the site of shorezone depositional systems throughout most of the Tertiary and has not been affected by salt mobilization or the sedimentological overprint of large delta systems. Consequently, rates calculated in the area represent average rather than extreme values. Detailed correlation of 142 well logs and the utilization of paleontologic data from 17 wells made possible the construction of a chronostratigraphic cross section which divided the Tertiary section into 1 or 2 million year increments. A decompaction computer program, which utilizes published compaction curves and distinguishes between normally pressured shale, underconsolidated overpressured shale, and sandstone, calculated original sediment thickness and sedimentation rates for each of the chronostratigraphic layers. Regional sedimentation rates varied almost two orders of magnitude during the Tertiary. These findings seriously challenge the common assumption of relatively uniform sediment supply over geological time spans. Within individual chronostratigraphic layers, sedimentation rates ranged from 4 cm/1000 yrs in the fluvial and transgressive facies to 130 cm/1000 yrs in the lower Miocene outer shelf and slope facies. Decompaction of the section also allowed the calculation of growth fault slip rates and of the percent increase in accommodation on the downthrown side of the faults relative to their upthrown equivalents. Slip rates during times of maximum movement ranged from 250-600 m/my, agreeing with rates calculated for salt dome growth. Percent increase in accommodation on the downthrown side of the faults ranged from 20-83%. Shelf-margin progradation rates were calculated and ranged from -1.5 mi/my (2.4 km/my) during brief times of temporary retrogradation (Queen City and Yegua Formations) to 17.5 mi/my (28.2 km/my) during the Early Miocene