Brian E. Lock

Flat-plate subduction and the Cape Fold Belt of South Africa

A flat-plate subduction model is proposed for southwestern Gondwanaland for late Paleozoic and Triassic time, in an attempt to explain some of the anomalous features of the Cape Fold Belt of South Africa and its continuations in South America and Antarctica. This fold belt is located at distances of as much as 1,000 km from the continental margin of the time, and it is suggested that the descending slab of oceanic lithosphere being subducted along the Andean margin of the continent became coupled with part of the overriding continental crust of Gondwanaland. Thus, some of the plate convergence at the boundary may have been taken up by deformational shortening within the Gondwana plate rather than by normal subduction at its edge. The deformation was partly concentrated along the intraplate Cape Fold Belt, which may have been situated over the zone where the subducted oceanic plate finally separated from the base of the continental lithosphere. Geological and geophysical data support the hypothesis, which should be tested further.

Southern Louisiana salt dome xenoliths: First glimpse of Jurassic (ca. 160 Ma) Gulf of Mexico crust

No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because the igneous rocks are deeply buried beneath sediments. Three salt diapirs from southern Louisiana exhume samples of alkalic igneous rocks; these salt domes rise from the base of the sedimentary pile and overlie an isolated magnetic high, which may mark the position of an ancient volcano. Three samples from two domes were studied; they are altered but preserve relict igneous minerals including strongly zoned clinopyroxene (diopside to Ti-augite) and Cr-rich spinel rimmed with titanite. 40 Ar/ 39 Ar ages of 158.6 ± 0.2 Ma and 160.1 ± 0.7 Ma for Ti-rich biotite and kaersutite from two different salt domes are interpreted to represent the time the igneous rock solidifi ed. Trace element compositions are strongly enriched in incompatible trace elements, indicating that the igneous rocks are low-degree melts of metasomatized upper mantle. Isotopic compositions of Nd and Hf indicate derivation from depleted mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern fl ank of the Gulf of Mexico ca. 160 Ma. These results also confi rm that some magnetic highs mark accumulations of mafi c igneous rocks buried beneath thick sediments around the Gulf of Mexico margins.

Xenolithic Inclusions Within the Salt at Weeks Island, Louisiana, and their Significance

ABSTRACT The salt seen in Gulf Coast mines is predominantly halite, with a few percent of anhydrite and traces of other evaporite minerals. Complexly folded salt structure is picked out by subtle compositional banding and interpreted as highly stretched bedding. Inclusions of non-evaporite rocks also tend to be distributed along the same banding. The non-evaporite lithologies, interpreted as xenoliths picked up by the salt during its rise to the surface, are composed mostly of sand sandstone and shale, highly brecciated and permeated by salt, and generally colored red, brown, orange or yellow. The color may be from oxidation of iron-rich mine waters. Fossils (Oligocene foraminifera) have been reported from only one location in the Five Islands mines, in a band of dark shale in the Belle Isle mine. Petrographic examination of several sand samples reveals well-sorted, well-rounded, coarse, frosted grains, suggesting a possible eolian origin. This interpretation would be most consistent with an origin within the Jurassic, either just above or even below the Mother Salt. Sub-salt inclusions have previously been documented from Persian Gulf domes. Two different serpentinized peridotite samples have also been recovered from the mine; these are interpreted as possible sub-salt lithologies, perhaps from mantle-derived intrusions along a fundamental fault responsible for the NW-SE Five Islands trend. This fault parallels a pattern recognized many years ago by Fisk and perhaps related to the early Mesozoic opening of the Gulf of Mexico.

Field Examination of Exposed Evaporite-Related Structures, United States and Mexico: Relations to Subsurface Gulf of Mexico Examples

ABSTRACT Considerable exploration attention has been devoted to salt-withdrawal minibasins and sub-salt traps in the offshore Gulf of Mexico (GOM) province. Investigation is inherently by geophysical and petrophysical means, utilizing sonar, seismic and gravity data, and scattered well logs, with obvious resolution limitations. Outcrop studies in Texas, Utah and Mexico provide an additional dimension, leading to an improved understanding of GOM examples. Outcrops along I-10 between Kerrville and Sonora, Texas, provide a remarkable suite of evaporite-withdrawal (dissolution) structures, including varying exposure levels of withdrawal synclines, related secondary structures, and the primary weld surface. Paradox Basin exposures in Utah include diapirs and evaporite anticlines, revealing intense internal deformation, and various stratigraphic-structural interactions. The La Popa basin in Mexico hosts evaporite-cored anticlines, diapirs, withdrawal synclines, and a previously-recognized secondary weld. These outcrop analogues provide excellent insight into GOM structures. For example, GOM welds form potentially important hydrocarbon pathways, but the coarse scale of seismic resolution, though revealing significant character changes, leaves many questions unanswered. The Edwards Plateau and La Popa welds exhibit remarkable diversity along their traces, even over relatively short distances. Some segments exhibit enhanced permeability through brecciation, fracturing, or sand-sand juxtaposition, whereas shale smearing or remnant salt limits fluid transmission along other portions.

Geologic Mapping of Salt Mines in Salt Diapirs: Approaches and Examples from South Louisiana

Abstract Gulf Coast salt diapirs have complex internal structure, which is shown by light and dark layering. The layering reflects variation in anhydrite content and is interpreted to represent highly stretched and sheared depositional bedding. Structure is dominantly vertical, consisting of repeatedly refolded isoclinal folds having vertical plunge. Geologic mapping is conducted primarily as an aid to mine planning and development; separate maps are produced to record basic structure and display the distribution of clastic stringers, sylvite stringers and lenses, crystal size, wet areas, and gas. The latter characteristics, including coarse salt crystals, are anomalous and tend to be associated (in anomalous areas and trends). Gas is a mining hazard, causing Weeks Island and Cote Blanche Island to be classified by MSHA as gassy mines having many associated regulations. Five miners died in a gas explosion at Belle Isle mine in 1979, and blasting is now normally done while the mines are unoccupied (before the morning shift goes in, for example). Water influx, particularly if derived from outside the mine, is a cause of major concern for obvious reasons; at Avery Island a crew is constantly drilling and grouting to maintain control of leaks, and Jefferson Island mine has been lost as a result of flooding when penetrated by an oil rig. Sand reduces the salability of the commercial product and also increases mining costs. The geologic maps provide structural trend information to help the mine engineers avoid undesirable or potentially dangerous areas including those in proximity to the edge of the diapir. They also cast light on the nature of salt diapirism; linear anomalous zones have routinely been interpreted (sometimes with minimal justification) as boundaries between separate salt spines. Map examples from Weeks Island are included.

Sedimentology of a modern point bar at raven camp on the Red River, central Louisiana

ABSTRACT The Red River Waterway is a Congressionally-funded project designed to establish a navigation channel from the Mississippi River to Shreveport. Channel straightening in the Raven Camp area, at River Mile 136, has artificially cut off a meander loop and lowered local base level, so that the point bar is almost completely exposed at low water stage in the late summer and fall, while winter and spring high waters still completely submerge the bar and reactivate sediment transport. Air photographs taken over the past twenty years record a surprising rate of channel migration and shifting sites of sand accumulation; these processes continue today, despite artificial constraints by the U. S. Army Corps of Engineers. As the spring flood waters recede, they expose a variety of sedimentary structures, dominated by sand waves over most of the bar surface. At the downstream end of the bar, where floodwaters rejoin the navigation channel, the sand waves have heights of up to two feet, although over most of the bar they average six inches; the crest to crest interval is between ten and fifteen feet. Climbing linguoid ripples cover the uneroded surfaces of the sand waves, and grade up to mud drapes which mark the last stages of individual floods. Large scale foreset bedding characterizes the bar margin adjacent to the low water channel, and well developed reactivation surfaces are common. During the stage of receding waters, symmetrical wave ripples having a wave length (crest to crest) of slightly more than one inch are formed by wind-generated waves in very shallow standing water close to the waters edge. Dropping water stages are recorded by convolute lamination and by internally chaotic beds of fluidized sand. These are structures that relate to dewatering of the sediment. Desiccation cracks and curled-up mudflakes are common. Many mud clasts are incorporated in subsequent flood-stage sands. After exposure, point bar sands are further modified by raindrop impacts and associated erosion and by the formation of rills and deep gullies during storm water run-off. During dry weather periods, eolian transport begins and texturally distinctive ripples form. Adhesion structures develop on damp sands close to the waters edge during eolian episodes. Biological traces are surprisingly common, and include the effects of rooted plants (particularly from the fast growing willow seedlings), vertebrate and insect traces, algal mats, and associated biogenic gas escape structures. Deposits of one winter/spring season are commonly scoured and removed the following year, followed by fresh deposition over a different part of the bar. Scour surfaces are abundant internal features of the point bar. Study of the Raven Camp point bar over a period of several years provides important lessons for the explorationist seeking hydrocarbon s in fluvial sands. Many of the sedimentary structures that might be seen in conventional cores are not ones traditionally regarded as indicative of point bar sands, and the reservoir heterogeneity and facies architecture are far more complex than much of the literature would suggest.

Very Early Supratidal Diagenesis in Washover Sediments, Puerto Penasco, Gulf of California, Mexico: ABSTRACT

Puerto Penasco, Sonora, Mexico is built on a macrotidal (tidal range up to 7 m) barrier complex. East of the town, a filled washover channel has been cut off from the sea by dune migration, but has experienced periodic inundation by marine waters - most recently as part of an aquaculture program. The climate is hot (mean August air temperature 30/degrees/C (86/degrees/F), mean annual rainfall 7.4 cm (2.9 in.)). Unconsolidated sand samples from core material were dehydrated with alcohol and impregnated with L. R. White resin, an extremely low viscosity agent used primarily by biologists. This resin is cured at 65/degrees/C (149/degrees/F), a temperature unlikely to significantly modify sensitive mineralogies over the short times involved. The method was extremely successful in preserving highly delicate microstructures such as micrite-filled endolithic algal borings within hollow micrite envelopes. Study of cored material from this sand body reveals a surprisingly complex diagenetic history, probably representing at most a few hundred years. Marine flooding episodes have introduced sulfates and chlorides; freshwater influx, perhaps following infrequent heavy rains, has led to partial dissolution of evaporites and redistributions of carbonate. Long time periods clearly are not a prerequisite for complex diagenetic changes in clastic sediments.

Resistivity Curves from Complex Reservoirs: Under-Utilized Tools for Exploration and Production Geologists: ABSTRACT

Most geologists regard resistivity curves simply as sources of information concerning lithology and degree of water saturation of subsurface strata; however, other important geologic information can also be obtained. For example, water saturation is related both to pore characteristics and to the buoyancy pressures that lead to water expulsion from those pores by a hydrocarbon column. Thus, resistivity response is indirectly modified by a complex of factors including pore size, pore geometry, and grain-surface characteristics (including presence of clays). The combined effect of these pore characteristics is revealed most directly by capillary pressure curves. Because these same factors also determine permeability, resistivity itself can be regarded as responding to perme bility. This approach to resistivity interpretation has several important consequences. For example, true oil-water contacts encountered in the well bore display transitional resistivity values as upwardly increasing buoyancy pressures approach those necessary to produce irreducible water saturation. Complex reservoirs with inclined permeability barriers (such as shale drapes along lateral accretion cross-bedding in point-bar sands or shale interbeds in tilted turbidite sand sequences) may include false water levels where vertically adjacent but separated beds may have different fluid contents and may lack transition zones. Recognition of the distinction between these two types of oil-water contact may profoundly affect reserve calculations and help avoid passing over of viable reservoirs.Correctly interpreted resistivity curves may also permit recognition of water production resulting from leading permeability barriers, may aid in distinguishing different types of oil show (leading, trailing, and residual) and may aid in explaining and predicting early water production, etc. Proper use of this readily available tool can have an important impact on successful exploration and production. End_of_Article - Last_Page 300------------

Marginal Marine Evaporites, Lower Cretaceous of Arkansas: ABSTRACT

The mixed evaporite/carbonate/terrigenous clastic sediments of the DeQueen formation, in southwestern Arkansas, were deposited at the End_Page 1467------------------------------ landward margin of a broad shallow lagoon formed behind the Glen Rose reef. About 60% of the sedimentary volume consists of mudstone, silt, and sand, with brackish-water to hypersaline ostracod faunas believed to result from influx of flood waters from the Ouachita highlands a few miles to the north. The lower part of the formation contains discontinuous beds of gypsum, ranging in thickness from a few centimeters to composite beds > 3 m (10 ft), and displaying mosaic structure with vertically oriented, elongate nodules. These beds, which are lenticular, are interpreted to result from subaqueous precipitation of vertical selenite crystals (subsequently recrystallized) in discrete ponds and pools on microtidal-range mud flats. Intrastratal growth of gypsum nodules and displacive hali e occurred at the margins of the pools. The upper part of the formation contains no gypsum beds, but halite pseudomorphs at the base of and within some of the thin limestones suggest the presence of supratidal brine pools. Several minor unconformities exist, of which one has a regional extent and is underlain by red-brown mudstones. Algal-mat lamination, lenticular gypsum pseudomorphs (an intrastratal growth form), and syneresis cracks occur in the limestones, and a supratidal environment is envisaged for a significant proportion of the time of deposition. The limestones generally have a restricted fauna of ostracods, bivalves, cerithid gastropods, serpulid worms, and miliolid Foraminifera, and range in texture from lime mudstones to grainstones. The most abundant grain types are pellets, superficial ooliths, and terrigenous quartz. A paucity of dolomite is a striking feature. Some of the thinner bedded units are rippled, and some ripples were truncated during periods of emergence. The limestones are believed to represent periods of shallow water, slightly hypersaline to slightly hyposaline conditions of variable energy. The regressive trend displayed by these two divisions continued with deposition of the overlying formation. The uppermost 3 m (10 ft) of the DeQueen consists of mudstone with a thin marl at the top. A conformable contac exists with the overlying Antlers formation, which has basal mudstones becoming more silt- and sand-rich upwards and finally giving way to the typical Antlers (Paluxy) sands. End_of_Article - Last_Page 1468------------

Toward a Better Understanding of Gulf Coast Miocene Deep-Water Sediments: ABSTRACT

Deep-water sands are known from many localities in the lowest Miocene of the Gulf Coast. They are recognized by a combination of paleontologic, conventional core, wireline log, and regional setting criteria. Almost invariably, subsurface geologic interpretations are difficult to make because of correlation problems. Turbidite deposition probably took place in deep-sea fan environments in most cases, but conventional models for these fans are over-simplified for application in studies of the Gulf Coast Miocene. The Miocene outer continental shelf and slope were not characterized by smooth, simple topography, but were extremely irregular, due principally to salt and shale tectonics. Thus the Walker and Mutti and Ricci-Lucchi fan model is inadequate and serves only as a star ing point for Miocene studies. Turbidite sand bodies in this setting can be classified into three major types: major channel deposits, levees, and suprafan lobes. In the Walker model, major incised channels with levees are characteristic of the upper fan, and suprafan lobes in the mid-fan, but in the Miocene they occur together in a complex manner. In the simplest cases we might anticipate that the channels cut through the lobes, resulting in rapid lateral facies change. Recommended procedure for mapping and interpretation involves preparation of panel diagrams after initial correlations have been made as completely as the data permit. Reduced-scale log segments are pinned on a large-scale base map, channel trends are identified, and the channel sands contoured, to the exclusion of other data. Lobe sand bodies are then contoured as a secondary operation and a composite sand or net sand isopach prepared. Abrupt changes in contour trend will occur where lobe and channel sands meet, and the two stage contouring process will yield much more accurate isopachs, and give a better tool for prediction of sand body extensions. End_of_Article - Last_Page 1433------------