Sequence stratigraphic framework and systems tracts analysis of Lower Miocene shelfal clastic deposits : Redfish Bay area, Texas Gulf Coast
Creators: name: Moore, Brian Timothy
Publisher: name: The University of Texas at Austin
Year: 2005
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Stratigraphy
subject: Sequence stratigraphy
subject: Miocene
subject: Sedimentary rocks
subject: Texas
subject: Redfish Bay

description: The deposits of the Lower Miocene section of the Redfish Bay area along the Texas Gulf Coast were characterized and their systems tracts and depositional environments were interpreted. A three-dimensional seismic volume and associated wireline-logs were used in an analysis of base-level changes and in the characterization of third-order sequences within the Lower Miocene stratigraphic section, which includes highstand, transgressive, and lowstand-incised-valley systems tracts. The Lower Miocene interval in the Redfish Bay study area consists of four unconformity bounded third-order sequences. The oldest sequence, sequence I, contains a lowstand systems tract (LST), transgressive systems tract (TST), and highstand systems tract (HST). The above-lying sequence II is similar to sequence I. The next two younger sequences in the Miocene interval, sequences III and IV, contain only the TST and HST. Emphasis has been placed on the LST incised-valley deposits in this study as the origin of the sand that makes up this system tract has been debated for years. LSTs contain a higher net-sand thickness to gross interval thickness ratio, relative to the TSTs and HSTs. There is a decreasing sand-percentage trend from LST to TST to HST in the lower two sequences. A decreasing sand-percentage trend from TST to HST continues in sequence III. In sequence IV there is an increasing sand percentage trend from TST to HST. The deposits of the LST of sequence I are interpreted as incised-valley fill. The incised-valley fill contains higher-frequency sequences, within the third-order LST, composed of amalgamated fluvial deposits and transgressive estuarine deposits. The TST of sequence I displays more wave-dominated and more wide-spread sand deposits than the LST. Depositional systems encountered in the HST are progradational deltas and interlobe deposits. The lowstand amalgamated fluvial deposits and transgressive estuarine sediments, which filled the incised-valley of the third-order LST of sequence II, were deposited during higher-frequency cycles. The interbedded stratigraphy of the high frequency sequences within this LST suggests that it has much more estuarine influence than the LST of sequence I. The deposits of the TST of sequence II are more dominated by destructive processes than the deposits in the LST. Depositional systems composing the HST are difficult to discern, but are interpreted as having been deposited in a shallow water, mud-rich environment. Within sequence III, no LST incised valley deposits are recognized from the wireline-log patterns or in the seismic data. It is also evident from the wireline-logs that the TST and HST in sequence III are very sand poor. The TST of sequence IV had more sand influx relative to sequence III. As indicated by wireline-log analysis, fluvial depositional systems dominate the TST and HST. The variability in the amount of amalgamation of fluvial channels within the systems tracts suggests that the fluvial system is responding to relative base-level fluctuations. In the TST, the fluvial channels range from more amalgamated sand bodies at depth where rate of relative base-level rise is lowest, to less amalgamated sand bodies with an increasing rate of relative base-level rise. In the HST where relative base-level was already high, the fluvial channel sand bodies become less amalgamated following the maximum flooding event. As the relative rate of base-level rise decreases in the HST, the fluvial channel sand bodies become more amalgamated. Integration of wireline-log characteristics, structural mapping from 3-D seismic data, isopach mapping between third-order erosional surfaces, and analysis of seismic stratal slices, demonstrate the existence of dip-oriented, incised-valley depositional systems within the LSTs of sequences I and II. The Lower Miocene stata in the study area were deposited twenty miles up-dip of the late Oligocene shelf-edge. The recognition of incised-valley systems is important for understanding the sequence stratigraphic framework and the origin of thick, massive sands of the Lower Miocene deposits of the middle Texas Gulf Coast. The incised-valley systems are also important because they transported Lower Miocene sediment to deepwater areas