E. William Behrens

Geology of a Continental Slope Oil Seep, Northern Gulf of Mexico

An oil gas seep was documented by replicate sampling with piston corer, abundant high-resolution and sparse multichannel seismic reflection profiling, and chemical and isotopic analyses. The seep occurs on the upper continental slope over a salt ridge interpreted to split and plunge eastward, northeastward, and northward. The relatively shallow diapir over which the seepage occurs is manifested at the surface by a graben in strike section and by a half-graben in dip section. Faulting over the crest is commonly associated with loss of reflected energy or acoustic wipeouts. Most cores taken in wipeouts with prolonged bottom echoes contain oil and gas. The cores also commonly contain secondary carbonate derived from microbial degradation of hydrocarbons. The isotopic lightne s of the carbonate and its negative correlation with porosity may be subtle indicators of seepage at sites where oil and gas are not obvious. The seepage demonstrates the existence of source rocks and maturation at this site.

Time of accumulation of hypersaline anoxic brine in Orca basin (Gulf of Mexico)

The Orca basin is an intraslope basin of 400 sq km area at a depth of ∼2400 m, on the continental slope in the northern Gulf of Mexico, containing 200 m of highly saline anoxic brine. The sedimentary units in a core from this basin are black mud from 0 to 485 cm with three turbidite beds of gray mud totalling 70 cm and gray mud from 485 cm to the bottom of the core at 1079 cm. It is interpreted that the black mud was deposited in a highly anoxic saline environment, and gray mud deposition took place in an oxic environment. The lack of sulfate reduction and bioturbation in the black mud, the occurrence of a manganese oxide peak in the gray mud, and geochemical differences between black and gray mud support this interpretation. The major black—gray boundary at 485 cm which has been dated by 14C to be 7900 ± 170 yrs represents the time when brine began to accumulate in the Orca basin and the depositional environment became anoxic. The interstitial water salinity in the core decreases from 238 g/kg at the top to 112 g/kg at the bottom, suggesting that the Orca brine did not diffuse from the sediments underneath but entered the basin from the surrounding slopes. The salinity profile in the sediments calculated for a downward-diffusing brine is similar to the observed profile and supports the beginning of brine accumulation at 7900 yrs B.P. Possible exposures of a salt surface to the sea water to the southeast and north of the basin are interpreted from two multichannel seismic profiles.

HIGH HEAT FLOW ANOMALIES OVER SALT STRUCTURES ON THE TEXAS CONTINENTAL SLOPE, GULF OF MEXICO

We made 74 closely spaced (< 2 km apart) heat flow measurements around and over two salt structures on the Texas continental slope, Gulf of Mexico. The values outlined the shape of the heat flow anomalies over both structures. Based on a preceding high resolution seismic survey, we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall-shaped feeder. The heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation. The values over the salt plug are nearly all greater than 70 mW/m². The measurements over the salt tongue have a sharp heat flow peak of 90 mW/m² associated with the presumed feeder and rather uniform values around 60 mW/m² over the remainder. The variation of heat flow over both structures is smooth and shows no apparent scatter. Heat flow values off these features are uniformly low, around 30 mW/m². Thermal effects from bottom water temperature fluctuation, slope sedimentation, diapiric movement of the salt body, and pore fluid migration appear unable to provide a satisfactory explanation for the observations. However, thickness variations of a highly conductive salt body can easily account for the heat flow anomalies. We suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt.

On sedimentation rates and porosity

Abstract Apparent sedimentation rate differences between core samples may be simply artifacts of porosity differences (a real example is provided). To avoid this problem, vertical thickness per unit time sedimentation rates (e.g. cm/1000 years) should be calculated on a uniform porosity basis. The relationships between porosity and mineral density, water density, bulk density, water contents, and salinity are reviewed; and simple, working equations for determining the necessary parameters are given as well as a choice of equally simple equations for reconstructing a core to a constant, reference porosity.

Unifite muds in intraslope basins, northwest gulf of Mexico

Uniformly structureless clayey muds, very much like those termed unifites or homogenites in the Mediterranean and other basins, occur in intraslope basins in the northwest Gulf of Mexico. Their organic carbon and carbonate contents indicate a terrigenous source. Their age (about 17,000 B.P.) approximates the time when large-scale slumping of terrigenous delta fronts formed the Mississippi Canyon (Trough). Their compositional dissimiliarity to nearby hemiplagic mud precludes a homogenite-like origin involving a tsunami. However, it is uncertain whether they are end-products of bypassing (unifites) or entrapments of entire flows of sandless clays from deltaic facies of the same composition.

Abyssal sediment waves in the Gulf of Mexico: An enigma

A sediment wave field in the western Gulf of Mexico is similar to many other fields that are located on continental rises and related to thermohaline contour currents, especially western boundary undercurrents. Such currents should not occur in the Gulf, because it is silled. At present, the only postulated source of deep currents in the Gulf is the Loop Current (in the eastern half), eddies detached from it, and Rossby waves but no thermohaline currents. If any of the postulated processes cause the mud waves, it is a new mode of formation. If they do not, much more needs to be learned about deep Gulf circulation. These questions are important, because the sediment waves represent paleoceanographic conditions that have persisted for over 5 million years.

Seismic evidence for timing of shallow salt emplacement on the continental slope

ABSTRACT The mechanics and timing of salt emplacement are important elements in the evaluation of potential hydrocarbon traps. In a mid-slope study area off Texas, several seismic sequences thicken (especially upslope) off several salt structures. Tentative correlations of two boundaries of such sequences with Sangamon and Trim A horizons indicate that the structures have been positive bathymetric elements (thus salt has been intruded) for at least 0.5 ma and have acted as at least partial dams to downslope sediment transport. Faults over the salt structures commonly terminate within sequences which expand off salt, indicating movement of the salt within the respective time intervals and little, if any, lag from the time of forcing sediment loading. Within the study area and upslope from it, salt structures are discreet and penetrate to within 200 - 300 meters of the water bottom. The salt surfaces are approximately symmetrically dome shaped in the strike direction (NE - SW), and often dip basinward (SE). Downslope and southward from the study area, salt appears predominantly as extensive flat surfaces (sheets or canopies?) at about 1 kilometer depth. The studied structures are interpreted to have originated as a wall of salt periodically reactivated by upslope sedimentation. Segmentation of the wall, further flow from depth, and/or salt crest subsidence lead to growth of tongues extending downslope from the original stocks.

Extrapolation of physical properties of sediments from a localized area in the Gulf of Mexico, based on a conceptual geological model

Abstract Conceptual models of depositional systems may permit extensive extrapolation of physical properties for a relatively small amount of detailed correlation among physical, sedimentological and acoustic properties. For example, in a 9-m midslope core (IG19-39, lat. 28°51.2′N, long. 87°14.0′W, depth 1006 m) in the northeastern Gulf of Mexico there are two approximately 60-cm zones (at the core top and from 750 to 810 cm) which have distinctly higher foraminiferal sand and clay contents than the rest of the core. In the foram-rich zones, the relative increase in both clay- and sand-sized particles leads to poorer sorting, lower silt/clay ratios, more negative skewness, less platykurtosis, and higher fine-clay/total-clay ratios than in the silty clay which comprises the rest of the core. Within the silty clay, bulk densities and in situ acoustic velocities increase with depth from 100 to 425 cm, remain at or near peak values to 700 cm, and then drop sharply into the lower foram-rich zone. Although high impedence contrasts within this sequence do not correlate well with the 3.5-kHz reflection pattern, the pattern is distinctive, and its extent suggests that the cores sedimentary pattern extends over about 4000 km2 on the northeastern continental slope of the Gulf of Mexico. The foram-rich zones have a predominance of warm-water species and correlate closely with higher carbonate content (in the mud as well as in the sand fraction) and more positive δ13C values (for organic carbon), which are typical of marine-derived organic matter. On the other hand, the silty clay has few if any warm water indicating forams and much less carbonate. It has organic carbon with more negative δ13C values, indicative of a terrestrial origin. Several investigators have concluded that, in the Gulf of Mexico, the foram-rich zones represent warm, interglacial or interstadial periods dominated by pelagic sedimentation in the deep-sea, whereas the siltier zones represent glacial periods of low sea-level stands during which more terrigenous sediment was delivered to the continental margins and transported to adjacent ocean basins by various gravity flow mechanisms. Since the causes of the changes in depositional processes are glacio-eustatic, the change in sediment types should be widespread with good lithostratigraphic correlations. Published data exist for over 100 cores from the deep Gulf of Mexico wherein only one or a few sediment properties are reported. In a large number of these cores the reported property correlates well with the pattern in the core studied and is explained by the glacio-eustatic model. This suggests that the relationships between physical, chemical, textural, and acoustic properties in core IG19-39 may be used to predict similar relationships over a widespread area. The model seems to apply to the abyssal plains of the Gulf of Mexico, the margins of the Mississippi Fan, and various parts of the northern continental slope.

Late Holocene Foraminiferal Biofacies from Baffin Bay, Texas: Preliminary Analysis of a 4,000-Year Record of Paleosalinity

ABSTRACT Predominant hypersalinity in Baffin Bay, Texas, leads to the preservation of laminated sediments. These largely unbioturbated sediments permit reconstruction of a high-resolution, late Holocene paleoenvironmental record. Piston cores from the center of Baffin Bay recovered up to 5 m (16.4 ft) of these fine-grained sediments. On the basis of previous dating of dolomite layers and sedimentation rate estimates (ca. 1 mm/yr), these cores represent up to 4,000 yr of deposition. Preliminary micropaleontological analysis of core L593-2 (4.9 m [16 ft] long) is based on 120 samples with an average sample spacing of 3-4 cm. Foraminiferal assemblages are dominated by three major taxonomic groups: (1) Ammonia parkinsoniana; (2) Elphidium spp.; and (3) miliolids (mostly Quinqueloculina spp. and Triloculina spp.). Previous work on modern microfaunas in Texas embayments shows that each of these groups may dominate benthic faunas under different environmental conditions. Miliolids dominate hypersaline environments, whereas the other two groups will tolerate moderately brackish water. Accordingly, core samples dominated by miliolids are interpreted to represent the most hypersaline conditions. Samples dominated by Ammonia parkinsoniana represent the most brackish salinities encountered in the core (but greater than 20 ppt). Dominance of benthic assemblages varies dramatically within the core. Major biofacies trends include (1) no occurrence of either low-salinity, agglutinated or open shelf assemblages; (2) peaks in miliolid abundance that define paleosalinity fluctuations on a scale of 10 to 30 cm in the upper 4.2 m (13.8 ft) of the core (thus representing cyclicity on the scale of hundreds of years); and (3) very rare or no miliolids from 4.2 to 4.9 m, suggesting an absence of hypersalinity, but still oceanographically restricted conditions. The core did not penetrate sediments deposited prior to restriction of Baffin Bay by Padre Island (ca. 5,000 yr B.P. Three dolomite layers within the core (at 2.86 m, 4.34 m, and 4.71 m) occur in intervals with rare or no miliolids, arguing against a hypersaline setting for their formation. Ongoing work on these sediments includes AMS carbon-14 dating to better constrain sediment accumulation rates and quantitative analysis of the benthic foraminiferal data by transfer functions, in order to derive quantitative paleosalinity estimates.

A large-scale intracoastal circulation pattern affecting estuarine—continental-shelf exchange

Abstract Monitoring of an artificial tidal inlet to Corpus Christi Bay, Texas, included continuous tidal recording at each end and calibrating the friction of the channel so that continuous discharges could be calculated from the Manning Equation. These revealed that twice as much water entered the bay through this channel on flood tides as left on ebb tides over most of a one-year period. Channels exist that could maintain this dynamic equilibrium by draining off wind setup to other intracoastal water bodies. The location of the inlet relative to the predominant wind mode and exchanging bay is the same as those of the natural tidal inlets of the Texas coast. This suggests that an anomalous flood excess could be maintained not only in this bay, but through an interconnected bay system with excess outflow at the downwind end of the system, i.e., Pass Cavallo at Matagorda Bay. Correlation of tidal flow and wind data, Corpus Christi Bay surface water slope, observations of interbay flows, and continental-shelf surface water particulate matter and salinity patterns support the proposed intracoastal circulation.