Sedimentary effects of differential sedimentation in Frio shoreface shelf sediments

Edwards 1986: Sedimentary effects of differential subsidence in Frio shoreface-shelf sediments, Gulf Coast Tertiary. Houston Geol. Soc. Bull. 29, 3, 10-14.

Index map for South Texas Frio study - 10k - click to enlarge

ABSTRACT

Sedimentologists and stratigraphers commonly invoke mechanisms such as changes in sea-level, subsidence rates and sediment supply to explain transgressive and regressive trends. In most cases, however, it is impossible to independently validate such interpretations. An important exception is the growth-faulted sedimentary sequences of the Gulf Coast Basin which were deposited at varying subsidence rates in closely juxtaposed locations. This short article presents some observations form the Oligocene Frio Formation of South Texas and suggests that subsidence rate is an independent parameter that influences reservoir character and geometry, and corresponding log and production characteristics. Approximately 600 well were correlated, and a series of detailed structural and stratigraphic cross-sections were compiled. Interpretations of stratigraphy and environment were based on study of electric log character.

INTRODUCTION

In general, the major growth-faulted zones of the Gulf Coast Basin correspond to the position of the contemporary shelf-edge (Winker and Edwards, 1983). Structurally. the shelf-edge is an extensional zone caused by the downslope creep of the muddy sediments on the continental slope. The extension results in rotated fault blocks, in which the strata often dip counter to the regional dip Despite the overall thinning which results from the extension, thereby enhancing subsidence rates, the strongly regressive character of these shelf-edge sequences indicates that sediment supply greatly overwhelmed subsidence. This would also have inhibited the development of substantial bathymetric relief. The considerable local variations in subsidence rate, both within and between adjacent fault blocks, resulted in corresponding variations in the thickness of sediments deposited during individual regressive sequences, These thickness variations are readily determined using paleostructural interval isopach mapping (Edwards, 1980)

Increasing cycles thickness with depth in Frio of South Texas - 42k - click to enlarge

DEPOSITIONAL MODEL

During Frio time, the study area (Figure 1) was the site of a wave-dominated shoreline, as suggested by both the strikewise continuity of stratigraphic units and the updip pinchout of all sandstones into marginal marine muddy sediments (Martin, 1969). A typical log through the entire Frio Formation (Figure 2) shows the gradual upward change from marine shales to shoreline sandstones, Several segments of the log (SP only is shown) are presented to show how sedimentary sequences (shown by the V's) become thinner upwards in the well. If it is assumed that regressive depositional cyclicity was caused by a process that had a roughly uniform occurrence through time, then it follows that the changes in thickness reflect changes in subsidence rates Thus, the lower part of the Frio was subjected to high subsidence rates consistent with the structural instability of the shelf-edge. As the regression proceeded, subsidence rates decreased with increasing structural stability.

Paleostructural dip section through Frio cycle - 77k - click to enlarge

Detailed stratigraphic sections illustrate another aspect in the relationship between contrasting thickness and log character of sequences. Section A-A' (Figure 3), datum below the regional "J" in local nomenclature. shows expansion across two major contemporaneous faults with modest rollover, whereas section B-B (Figure 4), datum below regional. At the other extreme, out on the shelf, wave action is low in local nomenclature emphasizes the effects of dramatic rollover expansion- Figure 3 emphasizes that in addition to thickening, increased subsidence results in a change in log character from digitate sand units updip to ratty serrated sand units downdip. This is present also in the stratigraphically lower unit shown in Figure 4. A core study (Berg and Powell, 1976) through the serrated sand facies from a nearby well illustrated the turbidite-like character of the sands.

The main problem is to explain the downdip change from blocky barrier-bar sands updip into an intermediate digitate facies and finally into a shelfal serrated sand facies downdip. In general terms, this can be accomplished with the standard regressive shoreface model (Mccubbin, 1982) which is diagrammatically shown in the upper panel of Figure 6. Progradation of the upper shoreface, a wave-dominated environment, and its associated inlet channels results in the updip facies (Galloway, 1986). Wave agitation retains finer sediment in suspension. Eventually, it is deposited in quieter water on the lower shoreface and shelf. While storms accomplish much damage and erosion of upper shore-face sediments, repair and renewed progradation occur almost immediately due to the frequency and efficacy of waves in this zone and infrequent, Sands are emplaced by high energy bottom flows that result from major storms. The exact nature of these flows, whether gravity driven, current driven or a combination, is still debated. In any event deposition of sediment during the waning of the flow gives rise to a turbidite-like deposit The alternation of sand deposition during storms, with mud deposition from both the waning storm flow and suspension during intervening quiet periods, gives rise to the interbedded serrated log character of this facies.

These contrasting forces, low energy high frequency waves and high energy low frequency storms, compete in the lower shoreface. Small-scale fluctuations in the local sediment supply in combination with the transition from the upper to the lower shoreface probably give rise to the intermediate digitate sand facies. Here, sands are part wave-deposited (mostly in the form of lenses in silt and mud deposited from suspension) and part storm-deposited (erosively-based sand beds). However, immediately adjacent. on the downthrown side of a large growth fault (Figure 5, lower panel), the digitate sands are somehow transformed into serrated sands, As there was no abrupt change in environment, the cause appears to be variations in subsidence rate In the lower shoreface, high subsidence rates in wave-influenced areas ensure that fine-grained deposits are preserved, by burial, from erosion by wave reworking. Similarly, high subsidence rates in storm-influenced areas ensure that the fine-grained end-of-storm-flow deposits and suspension deposits are preserved from erosion by large storm events.

Paleostructural section through Frio cycle - 84kk - click to enlarge

PRESERVATION POTENTIAL THRESHOLD

Following from these speculations is the concept of a preservation potential threshold. Deposits that result from a particular sedimentary environment in which competing erosional and depositional forces contrast in energy level and frequency, mayor may not reflect varying subsidence rates. It is only when the subsidence rate changes beyond a certain value or threshold, that the resulting deposit is influenced. While it is apparent that this 5 applicable to areas such as the Gulf Coast Basin where there are large changes in subsidence rate over short spans of distance and time, it must also be pertinent to the interpretation of environment in basins that had varying subsidence rates though a diverse tectonic history. Inversely, it seems likely that sediment supply has an analogous effect The same environment can result in totally contrasting sedimentary sequences depending on the sediment supply rate.

CONCLUSION

As basin analysis becomes more powerful and quantitative, perhaps these lines of reasoning may provide a route for further understanding and research. MeanwhiIe, in the Gulf Coast, "What's happening on the other side of the fault?" is still an important and valid question, whose answer should never be taken for granted. Other settings characterized by differing sets of sedimentary processes, such as river-dominated deltas, and gravity-influenced delta front slopes will illustrate their own unique subsidence-rate relationships to sedimentary and log attributes.

REFERENCES CITED

Berg R. R. and Powell, R R., 1976, Density-flow origin for Frio reservoir sandstones Nine Mile Point Field, Aransas County, Texas. Trans Gulf Coast Assoc. Geol. Socs v.26, p. 310-319

Edwards, M. B.,1980, The Live Oak delta complex: an unstable shelf-edge delta in the deep Wilcox trend of South Texas: Trans. Gull Coast Assoc. Geol. Socs. V 30, p.71-79.

Edwards, M. B., unpublished, Stratigraphic and Structural Analysis of Growth-Faulted Regions Using Well Logs - A Workshop. Course Notes.

Galloway, W E., 1986, Reservoir facies architecture of microtidal barrier Systems. AAPG Bull. v.70. p.787-808

Martin, C. B., 1969, The subsurface Fr?o of South Texas stratigraphy and depositional environments as related to the occurrence of hydrocarbons: Trans. Gulf Coast Assoc Geol Socs v.19, p 489-499.

Mccubbin, D. G.. 1982, Barrier-Island and Strand-Plain Facies: in Sandstone Depositional Environments, AAPG Mem. 31. p.247-279.

Weise, B H. et al., 1981, Geologic studies of geopressured and hydropressured zones in Texas: Test-well site selection. Final report prepared by the Texas Bureau of Economic Geology for the Gas Research Institute under contract number 5011-321-0125.

Winker, C 0. and Edwards, M. B., 1983, Unstable progradational clastic shelf margins: SEPM Spec PubI. No 33, p. 139-157.

ACKNOWLEDGMENTS

This paper, is based on an original study (Weise. et al. 1981) that examined the stratigraphy and structure of the Frio Formation in selected areas of the Texas Gulf Coast. That study was funded by the Gas Research Institute in order to locate reservoirs suitable for unconventional energy production. I acknowledge helpful discussions with Bonnie Weise and assistance from Don Downey, Susan Hallam, Steve Mann, and Doug Wilson.

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