Abstract

Sediment accumulation and net-sand maps drawn from a data set containing thousands of wells record the shifting loci of reservoir-prone third-order sequences and clarify onlap/offlap cycle relationships within the Miocene and lower Pliocene of the northern Gulf. Two second-order supersequences, one in the lower Miocene, bracketed by the Lenticulina jeffersonensis (Lentic jeff) and Bigenerina humblei (Big hum) sequence boundaries, and one in the middle Miocene - lower Pliocene, bracketed by the Big hum and Ceratolithus acutus sequence boundaries, contain, respectively, six and seven third-order depositional sequences. Accumulation rate patterns at the supersequence level reflect depositional trends related to globally manifest changes in continental erosion and may also identify regional climate and drainage system changes. Accumulation rate patterns at the sequence level record more local effects of river avulsion and salt movement. Sand is nearly equally apportioned between the Louisiana and Texas shelves within lower Miocene third-order sequences and unequally distributed within middle Miocene - lower Pliocene sequences. By the middle Miocene, the Mississippi River system had begun delivering significantly more sand and other sediment to the northern and eastern Gulf than the combined Texas rivers. This change may imply that the drainage system of the Red River was captured and included within the Mississippi River watershed, significantly reducing the source area of Texas rivers. Further reduction of sand deposition on the Texas shelf, during the implied sea level rise portion of the middle Miocene - lower Pliocene supersequence may have been in response to increasing aridity in Texas. The implied sea level fall portions of the supersequences are marked by smaller shelf-edge sand depopods and correspondingly enhanced down slope sediment transport to slope and abyssal plain depocenters. In Louisiana, sand depopods show significant onlap landward from the areas of supersequence lowstand deposition, whereas the Texas section generally progrades forming successive coast-parallel bands of narrow sandy depopods.