ABSTRACT

The Eocene Cole barrier bar-lagoon system in Duval County, South Texas, represents a minor depositional episode in the southwest Gulf of Mexico region. Lithofacies interpretation using wireline logs delineated lithofacies cycles consisting of transgressive shoreface and barrier-bar sandstone and shale in the lower part and regressive lagoonal shale and sandstone in the upper part. Each cycle represents a bar-building episode. Three orders of cycles, termed major, intermediate, and minor, indicate three scales of bar building. Facies mosaics of individual cycles are interpreted from facies geometry on cycle thickness maps and log facies patterns. Dynamic evolution of the barrier bar-lagoon system is interpreted within the cyclostratigraphic framework.

The Cole system as a whole exhibits a typical shoreline-parallel facies mosaic formed in a wave-dominated regime. However, facies mosaics of three major cycles indicate that the lower and upper cycles were deposited in a wave-dominated regime, whereas the middle cycle was deposited in a tide-dominated regime. The wave-dominated barrier bar-lagoon systems have large lagoons with well-developed washover fans and broad, continuous, and thick barrier islands with abundant tidal inlets. The tide-dominated system has well-developed shoreline-perpendicular tidal channels and flood and ebb deltas and highly dissected narrow barrier bars. Intermediate cycles are the building blocks of major cycles, which were generally formed in regimes similar to the corresponding major cycles. However, a tide-dominated system was formed in an overall wave-dominated regime. Minor cycles in some intermediate cycles were deposited in wave-dominated regimes and accreted seaward.

Bar building in the wave-dominated regime occurred in four stages - initiation, aggradation, progradation, and cessation. Small bars are initiated from subaqueous shoals, such as tidal

End_Page 510------------------------

ridges and relict bars or by spit accretion. They expand in width and height by aggradation of intermediate cycles, which become thinner and more uniform in thickness upward, probably owing to increased surface area. Once a bar attains a certain height, progradation of minor cycles occurs either seaward or alongshore, effectively expanding the barrier core. Finally, bar building ceases because of diminishing sand supply and/or transgression. Bar building in the tide-dominated regime is progressive destruction of contemporary shoreline-parallel bars through sediment dispersal by tidal currents, causing landward shift of subenvironments. As a result, the stratigraphic architecture of the Cole barrier bar-lagoon system is the superimposition of both wave-dominated shoreline-parallel systems and tide-dominated shoreline-perpendicular systems of three scales.