Henry L. Berryhill

Late Quaternary Shelf-Margin Deltas, Northwest Gulf of Mexico

Interpretations of 35,000 km (21,900 mi) of single-channel, high-resolution, seismic profiles traversing the continental shelf and upper continental slope of the northwest Gulf of Mexico indicate the existence of five late Wisconsinan shelf margin deltas, including the Rio Grande and Mississippi deltas. The deltas were recognized by geomorphic pattern, high-angle clinoform seismic reflections, and association with buried river systems. Isopach patterns show that the deltas range in size up to 5,000 km2 (1,900 mi2) and reach thicknesses of over 180 m (590 ft). The deposits are elongate parallel with depositional strike, indicating subsidence of the shelf margin as a whole. Internal reflection patterns show the deltas to be fluvially dominated. Multilo ate structure resulted from both short-term eustatic sea level fluctuations and delta switching. The late Quaternary shelf-margin deltas provide models for analogous deposits in the ancient record. They are primary indicators of the position of ancient shelf margins, and are important for predicting sand occurrence in that environment as well as farther downslope. As exploration moves to the shelf edge and beyond, instability hazards posed by late Wisconsinan deltas, as well as older deposits, must be understood and dealt with.

Late Quaternary Shelf Margin Deltas, Northwest Gulf of Mexico: ABSTRACT

Deltaic deposition during a eustatic sea level fall occurs in two phases, each controlled primarily by the morphology of the sea floor and the rate End_Page 532------------------------------ of subsidence. The initial, or shelf phase of deposition, produces deposits similar to most modern deltas, although the rates of progradation are enhanced by the cumulative effects of sedimentation and absolute sea level fall. Such deposits are relatively thin and widespread, and internally are characterized by low angle clinoform reflections. The second, or shelf margin phase, results when sea level reaches the shelf edge and deposition occurs on the upper continental slope, where steeper sea floor gradients and more rapid subsidence produce a more localized deposit. Interpretations of over 35,000 km of single channel high-resolution seismic profiles of the continental shelf and upper continental slope of the northwest Gulf of Mexico indicate the existence of 5 late Wisconsinan shelf margin deltas, including the ancient Rio Grande and Mississippi deltas. The deltas were recognized by geomorphic pattern, high angle clinoform seismic reflections, and association with buried river systems. Isopach patterns show that the deltas range in size up to 5,000 km2 (1,930 mi2) and reach thicknesses of over 160 m (525 ft). The deposits are elongate parallel to depositional strike, indicating subsidence of the shelf margin as a whole, as well as reworking by marine processes. Internal reflection patterns show the deltas to be fluvially dom nated. Multiple lobes can be recognized in most of the deltas studied, resulting both from short term eustatic sea level fluctuations and delta switching. End_of_Article - Last_Page 533------------

Environments of Sand Deposition, Southwest Louisiana Continental Shelf: ABSTRACT

ABSTRACT A synthesis of approximately 20,000 km of high resolution seismic profiles, coupled with vibracores and industrial platform borings, shows that a variety of large sand bodies occur in the late Pleistocene and Holocene sediments of the southwest Louisiana continental shelf. Patterns of sand deposition have been largely controlled by glacio-eustatic sea level fluctuations, paleogeomorphology, subsidence, and salt tectonism. Sand deposits of the area fall into two categories: (1) those associated with sea level lowstands, or regressive deposits; and (2) those associated with rising sea levels, or transgressive deposits. Regressive facies include fluvial and deltaic sands, while transgressive sands are largely formed by the reworking of regressive deposits.

Environmental Geology of Continental Margin Off Central and South Texas in Atlas Format: ABSTRACT

Integrated environmental studies keyed to understanding fundamental processes and their interrelations were conducted from 1974 through 1977. Most of the aspects investigated were sampled seasonally to provide data on variations within each year and among the 3 years studied. The geologic and related hydrographic, biologic, and chemical data have been compiled in an atlas format consisting of topical overlays so that interrelations are apparent. Three atlases, each covering a geographic area of 2° × 1°, have been completed: Port Isabel, Corpus Christi, and Beeville. Each, in addition to bathymetry and the lease-block grid, consists of seven topical maps: location of oil and gas fields and production installations; water circulation and rates of sedimentation; trace-metals content and texture of surficial bottom sediments (plus seasonal variations); nature of shallow subsurface sediments and biogeology; post-Wisconsin sedimentation patterns and tectonism; late Pleistocene and Holocene depositional environments; and structure of the continenta terrace with emphasis on the chronology of faulting and seafloor stability. Results of the study indicate that water circulation is strongly seasonal, but both local variations and differences in surface and bottom movement are evident. Sedimentation rates have been relatively high throughout the Holocene, averaging almost 1 m/1,000 years; rates now are as much as 9 mm/year locally, based on 210Pb dating. Seasonal differences in both the texture and trace-metals content of surficial bottom sediments are characteristic and probably can be related to variations in infaunal activity and movement of bottom sediments. Faulting has migrated progressively seaward across the shelf with time; movement during the Holocene has been mainly near the shelf edge. Slumping has been extensive along parts of the outer shelf and on the upper continental slope. End_of_Article - Last_Page 420------------

Criteria for Recognizing Ancient Barrier Coastlines: ABSTRACT

Worldwide modern barrier coastlines constitute a minor part of the total coastlines of all the continents. The aggregate length of present barrier coastlines in the world is approximately 3,530 mi, distributed as follows: North America, 2,000 mi; Europe, 500 mi; South America, 350 mi; Africa, 300 mi; Australia, 200 mi; and Asia, 200 mi. Barrier islands commonly border coastal plains adjacent to broad continental shelves. They form in areas of abundant sand accumulation where longshore currents are prominent. Sandstone lenses which represent ancient barrier islands would be expected in thick wedges of interfingering terrestrial and marine sandstone, siltstone, and mudstone. Barrier islands of Pleistocene age have been recognized inshore from present End_Page 706------------------------------ coastlines, and drowned Holocene barrier coastline features have been described on the continental shelves. Pre-Holocene linear sandstone bodies resembling barrier islands have been described in ancient rocks of Pennsylvanian, Cretaceous, and Tertiary ages. Probable barrier island sandstone bodies in ancient rocks have been described by previous investigators on the basis of comparison with features of modern analogs: geometry, sedimentary structures within the sand lens, physical properties of the sand, and the nature of associated environments. Recognition criteria used in this report are based partly on previous work and partly on recent studies along the Texas and North Carolina coasts. Barrier islands are linear, have a length to width ratio generally greater than 10:1 and commonly are less than 60 ft thick. Padre Island, Texas, consists of four morphological units that have characteristic sedimentary structures: beach, foredune, barrier flats, and wind tidal flats--though the development of the foredunes and wind tidal flats changes considerably from north to south. Along the North Carolina coast, wind tidal flats are absent, but accretionary beach ridges are locally prominent. Superimposed on the islands of both coasts are storm washovers of hurricane origin that breach the foredunes and channel inlets that cross the island and connect the sea with the lagoon behind the islands. Beaches contain laminae of different thicknesses that dip principally seaward; the san is locally shelly and fine laminae of heavy minerals may be prominent. The foredunes are markedly cross-bedded in an oriented pattern that reflects strongly the predominant wind direction. Barrier flats are underlain by sand which ranges from structureless to highly laminated; vegetal remains are common. Wind tidal-flat sediments that border the lagoon are an interlayered mixture of sand beds containing some fine shell fragments, and laminae of clay and algal remains. Sand is fine grained throughout. However, shell fragments, locally abundant, exhibit greater variability in size, shape, and sorting. Sand which refills channel inlets ranges from horizontally bedded to structureless; this contrasts sharply with the cut-and-fill cross-bedded sand common in stream-channel deposits. The associated lagoon sediments are organic and calcareous mud which interfingers with barrier-island sand; the fauna is less diverse than that of the open sea and unbroken shells are abundant. Tongues of sand--washover deltas and fans which are built by storm flood tides--are prominent local features of the lagoons. Marshes overlying peat are characteristic of the inshore side of the bays along the North Carolina coast. The geometry and alignment of the barrier islands and the close association of the sand in the barrier island with the organic mud of the lagoon are the key factors for the recognition of a barrier coastline. Attendant washover deltas and fans, cross-cutting inlet fill, and associated biota are important supplementary aids. End_of_Article - Last_Page 707------------