Baffin Bay, Texas is the flooded Last Glacial Maximum incised valley of the Los Olmos, San Fernando and Petronila Creeks along the north-western Gulf of Mexico. Cores up to 17 m in length and high-resolution seismic profiles were used to study the history of Baffin Bay over the last 10 kyr and to document the unusual depositional environments within the valley fill. The deposits of the Baffin Bay incised valley record two major and two minor events. Around 8AE0 ka, the estuarine environments backstepped more than 15 km in response to an increase in the rate of sea-level rise. Around 5AE5 ka, these estuarine environments changed from environments similar to other estuaries of the northern Gulf of Mexico to the unusual suite of environments found today. Another minor flooding event occurred around 4AE8 ka in which several internal spits were flooded. Some time after 4AE0 ka, the upper-bay mud-flats experienced a progradational event. Because of its semi-arid climate and isolation from the Gulf of Mexico, five depositional environments not found in the other incised-valley fills of the northern Gulf of Mexico are found today within Baffin Bay. These deposits include well-laminated carbonate and siliciclastic open-bay muds, ooid beaches, shelly internal spits and barrier islands, serpulid worm-tube reefs and prograding upper-bay mud-flats. Based on these unusual deposits, and other characteristics of Baffin Bay, five criteria are suggested to help identify incised valleys that filled in arid and semi-arid climates. These criteria include the presence of: (i) hypersaline-tolerant fauna; (ii) aeolian deposits; and (iii) carbonate and/or evaporite deposits; and the absence of: (iv) peat or other organic-rich deposits in the upper bay and bay-margin areas; and (v) well-developed fluvially dominated bayhead deltas.
The influence of tectonic control is more apparent than eustatic control on the rift-related stratigraphy of the Dampier Sub-basin. The correlation of observed depositional events to causative processes and global events is problematic due to the use of alternative geological time scales, causing ambiguity and uncertainty. The Harland (1989) time scale with a revised palynological allocation, combined with genetic sequence stratigraphy, and Prosser's concept of the tectonic systems tract, has proved useful during evaluation of the stratigraphy of the Dampier Sub-basin.Palaeo-topography was a major factor in sediment distribution and facies architecture of rift-related strata in the Dampier Sub-basin. This must be considered when assessing the stratigraphic trapping potential for hydrocarbons. There is a close association between the styles of depositional systems observed in the Dampier Sub-basin and stage of rifting and basin development. Five tectonic systems tracts, each with unique depositional systems have been identified and described; pre-rift, rift initiation, rift climax, immediate post-rift and late post-rift tectonic systems tracts.The use of a single time scale has enhanced the relationship between tectonic systems tracts (super-cycles) and the timing of depositional events recorded during previous genetic stratigraphic studies in the North West Shelf. The tectonic nature of super-cycle scale events should be temporally and spatially assessed in detail before the effects of eustatic change are evaluated for rift-related successions of the North West Shelf. The problem can be further exacerbated when the absolute error of chronological dating exceed the temporal frequency of eustacy, causing tenuous correlations of depositional events to a global eustatic curve.
A possible submarine fan system of Valanginian age occurs in the south of the western half of Permit WA-212-P in the Browse Basin. Seismic mapping and interpretation have allowed the recognition of five seismic facies which are considered representative of this fan system.The five seismic facies are the upper-middle fan braided channel facies, the upper-middle fan braided interchannel facies, the lower fan channel facies, the lower fan sheet facies, and the lower fan lobe fringe facies. The reflections of the upper-middle braided channel fan facies are discontinuous, disrupted, convex up, low amplitude and high frequency. The interchannel facies has reflections that are concave up, continuous, low frequency and moderate to high amplitude. The lower fan channel facies are recognised by convex up, discontinuous, high frequency and low amplitude reflections. The lower fan sheet facies is noted by mounded configurations with continuous, moderate to high amplitude, moderate frequency reflections. The lower fan lobe fringe facies reflections are flat, often shingled reflections with moderate discontinuity, moderate to high amplitude and low to moderate frequency.Since no wells penetrate the submarine fan, the interpretation is based on seismic reflection configurations which are considered typical of submarine fan segments. The interpreted ancient submarine fan occurs on the basin floor adjacent to a probable ramp type margin, and manifests shape and setting consistent with known submarine fans, such as the Eocene Frigg Fan of the North Sea, and the Lower Cretaceous Barrow Group turbidites in the Carnarvon Basin.Seismic facies mapping, in conjunction with sequence stratigraphy concepts, is particularly useful in areas such as the Browse Basin where considerable marine shale sections exist with little structure, and sequences with reservoir potential continue to be a problem to locate. For instance, in Caswell-1, a well drilled in 1977 immediately north of the permit area, 200 barrels of oil flowed from thin sands within a shale sequence of Albian age. Results of this study indicate that local seismic reflection signatures may be indicative of potential sandstone reservoirs in the vicinity of the study area.
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