David K. Davies

Effect of a large larval settlement and catastrophic mortality on the ecologic record of the community in the death assemblage

Abstract The death assemblage is an important source of data on the living community prior to sampling and between sampling occasions. Proper use of these data requires a knowledge of how death assemblages form from living communities. A large larval settlement and catastrophic mortality of the clam Mulinia lateralis provided an opportunity to test several hypotheses about death assemblage formation. Individuals of M. lateralis were added to the death assemblage in pulses resulting from episodic mortality, but each pulse rapidly disappeared—half of the shells were gone in 192 days. This loss was size-dependent; smaller shells disappeared at over twice the rate of larger shells. The size-frequency distribution for M. lateralis in the death assemblage was bi-modal because of the simultaneous death of two cohorts of different size frequencies. Individuals present in the death assemblage prior to the new settlement had been locally redistributed by physical processes after death. We hypothesize that larger pulses are preserved preferentially because they remain in the death assemblage longer, and, thus, these shells have a greater chance of being physically reworked beneath the taphonomically-active surface zone by storms. Consequently, death assemblages provide two distinct types of data; data on recent recruitment and mortality provided by pulses in the process of decay, and data on longer-term events provided by the accumulation of the remains of pulses that were buried beneath the surface zone and indefinitely preserved.

Dispersal of Mississippi Sediment in the Gulf of Mexico

ABSTRACT Pleistocene and Recent Mississippi sediments possess a distinctive heavy mineral assemblage which retains its identity between Cairo, Illinois and the Gulf of Mexico Abyssal Plain. Thus this assemblage may be used to trace the Mississippi contribution to the Gulf of Mexico from fluvial, through deltaic, neritic and bathyal, to abyssal environments. Significant changes in the heavy mineral assemblage of sediments in the Gulf are related to source changes and not to the reworking or selective sorting of Mississippi sediments. As a result, three distinct sediment input sources may be recognized for detrital sediments in the Gulf of Mexico Abyssal Plain, 1) The Mississippi, 2) the Rio Grande, and 3) the rivers of north-east Mexico. The Mississippi contribution is dominant and is only repl ced by other inputs in the northwest and southwest corners of the abyssal plain. On the Louisiana-Texas Inner Continental Shelf, Mississippi sediment forms a veneer which extends between the present delta and the Sabine River. Dredge samples reveal that underlying sediments were derived from the central Texas rivers to the west, probably during a period of regression which occurred between 10,000 and 7,000 B.P. The interaction of a high zircon content and intense selective sorting in the Inner Continental Shelf sediments has resulted in two areas of zircon enrichment which may be of economic significance. Because of the insensitivity of the heavy mineral assemblage of the Mississippi contribution to processes of selective sorting and reworking, only 200 non-opaque grains from one size fraction of one sample are needed to characterize this contribution.

Shelf sedimentation; an example from the Jurassic of Britain

ABSTRACT The refined biostratigraphic framework that has been developed for Jurassic sediments of northwest Europe enables precise environmental distinctions to be made on a time-equivalent basis. Using such control, it can be demonstrated that shelf sedimentation in the Lower Jurassic (Toarcian) of southern England was dominated by the gradual migration of a large sand bar. Despite the general homogeneity of its detrital sandstones and siltstones, this bar may be subdivided into four principal environments of deposition which include 1) bar (beach and upper shoreface), 2) fore-bar (middle shoreface), 3) back bar, and 4) tidal channel. These environments are distinguished on the basis of qualitative and quantitative textural and compositional criteria: succession of sedimentary structures, bio urbation, grain size, sorting, skewness, and petrography. The bar complex is 150 ft thick, and is transitional downward and southward into condensed limestones and argillaceous siltstones (40 ft average thickness). The limestones are principally intramicrites, the intraclasts being of the same composition as the host sediment (biomicrite), and often display oxidation rims. Such deposits are considered to be of shallow, marine origin. On the other hand, the argillaceous siltstones, which may contain up to 20 percent micrite, represent deeper marine conditions of deposition. The bar complex is succeeded upward and to the north by condensed limestones of variable composition, principally oomicrites, pisomicrites, and biomicrites. These limestones (12 ft average thickness) represent a platform interior sand blanket which was developed under shal ow marine conditions in the lee of the large bar complex. Because 1) new sediment was constantly added to the southward side of the bar, and 2) subsidence did not quite keep pace with sediment supply, the bar was forced to accrete continually in a southward direction, at a rate of 1 mile per 80,000 years. The gradual southward migration of the bar resulted in preservation of vertical sequences which, when completely developed, comprise a basal argillaceous and calcareous facies (the Upper Lias Clay Formation), succeeded by an arenaceous facies (the Upper Lias Sand Formation), and capped by a calcareous facies (the Cephalopod Bed Formation). Sediments comprising these three facies were deposited in two distinct basins of deposition, separated by an east-west trending structural high (the Mendip Axis). Both basins subsided to receive a maximum thickness of some 340 ft of marine sediments. Because of the southward migration of the bar, the southern basin received its major sediment input only after the northern basin had already been filled. The nature and rate of sediment supply remained constant throughout the 6 million years duration of the Toarcian, the principal source being a meta-igneous complex located in the present western approaches to the English Channel.

Pore Space Reduction in Cretaceous Sandstones through Chemical Precipitation of Clay Minerals

ABSTRACT Volcaniclastic sandstones from the Upper Cretaceous Horsethief Formation, Montana, are characterized by two distinct cement assemblages; (1) Corrensite ± Calcite ± Dolomite, which is restricted to samples from delta distributary channel and distributary mouth bar environments, and (2) Montmorillonite ± Calcite, which characterizes bay-beach, crevasse splay, lagoon, barrier island, and shallow subtidal environments. Petrographic, microprobe, scanning electron microscope, and fluid inclusion analyses indicate the early stage origin of these assemblages as authigenic chemical precipitates. In Horsethief volcaniclastics, pore-space reduction by precipitation of the clay minerals corrensite and montmorillonite reflects the chemistry of early stage pore-fluids, and does not involve syn- or post-depositional introduction of detrital clays.

Origin of Friable Sandstone-Calcareous Sandstone Rhythms in the Upper Lias of England

ABSTRACT The diachronous Upper Lias Sands of southern England are comprised of small scale rhythmic alternations of friable, yellow sandstone and firm, gray, calcareous sandstones. In general, the alternating layers are parallel to bedding and to one another; exceptions occur where the calcareous sandstones mark out the bases of erosional structures. Individual carbonate-rich or deficient horizons cannot be correlated between outcroups 2-3 miles apart, but within a single outcrop they preserve a remarkable uniformity in vertical sequence. Petrographic studies demonstrate that original textural relationships are retained in the calcareous sandstones, while the friable sandstone interbeds have undergone compaction effects. Field evidence indicates that the calcareous sandstones were lithified be ore the deposition of more than 6 inches of superimposed sediment. Such lithification is estimated to have taken approximately 150 years. The alternations are considered to be the result of fluctuating rates of detrital deposition: the calcareous sandstones representing temporally, and spatially restricted locales characterized by little or no detrital deposition, and the sandstone interbeds representing contiguous areas of more rapid detrital deposition. Within the Upper Lias Sands a gradual increase is observed in the relative proportions of CaCO3 to detritals. This is believed to reflect the southerly lateral migration of the wave of maximum terriginous sedimentation. This process established conditions suitable for deposition of the succeeding limestone sequences an environment foreshadowed by the calcareous sandstones of the Upper Lias Sands.

Porosities, Permeabilities, and Microfabrics of Devonian Shales

Shales generally are regarded as source rocks and seals. However, in the Illinois, Michigan, and Appalachian Basins, shales of Devonian age commonly are regarded as reservoir rocks. According to Broadhead et al. (1982), economic gas production from nonfractured shales has been an established fact in these basins for several decades. In some areas, natural fractures are an important control on production. In other areas, gas productive shale intervals lack natural fractures.

Fault-block origin of the western Cayman Ridge, Caribbean Sea

Abstract The western end of the Cayman Ridge is bounded by linear scarps to north and south. Analysis of seismic reflection profiles indicates that this portion of the Ridge is a fault block uplifted some 1500 m above the Yucatan Abyssal Plain to the north and the Bartlett Trough to the south. An asymmetric positive magnetic anomaly of 300 gammas is associated with the southern margin of the Ridge and exhibits a steep gradient to the north. Its source is interpreted as an igneous intrusion within the block, emplaced prior to or during faulting. This intrusion has a steep northern boundary located within the fault block, and may crop out along the southern slope of the Ridge in this region. Topographic evidence for the westward continuation of the Cayman Ridge into British Honduras is lacking. A piston core from the northwestern slope of the Ridge revealed 350 cm of Globigerina ooze overlying partially cemented, fossiliferous volcanic ash. The ooze is of varied age (Oligocene-Recent) and the faunal assemblages are mixed, indicating slumping. The fossiliferous ash is of Early Pleistocene age and may represent a blanket of sediment over the Ridge, rather than an intrinsic portion of the Ridge.

Dimensions and quality of reservoirs originating in low and high sinuosity channel systems, Lower Cretaceous Travis Peak Formation, East Texas, USA

Abstract Quantification of the dimensions and quality of fluvial reservoirs requires knowledge of channel style, depositional environment and diagenesis. Two styles of fluvial channel co-existed during the rapid aggradation of >600 m of Lower Cretaceous, Travis Peak sediments in East Texas: (1) high sinuosity (meandering) channels, and (2) low sinuosity (straight) channels. Each was restricted to a specific geographic area, the result of long-term, geomorphic stability. In both channel systems, reservoir sandstones originated principally in channel and crevasse splay environments. Reservoir models developed through sedimentological analysis are similar to those developed independently through reservoir simulation studies. Reservoir sandstones originating as point bars in high sinuosity channel systems are relatively small (1.2 km2 or less), thin (3.6 m), heterogeneous, and isolated within non-pay mudrocks. Reservoir sandstones originating as medial and transverse/oblique bars in the low sinuosity system are areally extensive (>20 km2) thick (3.6 to 13.7 m), homogeneous, and display complex pressure relationships due to avulsion of long stream segments, and the lateral and vertical stacking of successive channels in well-defined channel belts. The greatest volume of channel sandstone occurs in the low sinuosity channel system. The high sinuosity system is dominated by overbank deposits. Travis Peak sandstones have been extensively modified by compaction and cementation. Despite extensive diagenesis, permeability values reflect original depositonal environment and bedding style, even in rocks which have lost more than 80% of their original, depositional porosity. Channel sandstones have higher permeability than associated splay sandstones. Within a channel sandstone, the highest values of permeability occur in destratified, and flat- to low-angle cross-bedded sandstones: planar cross-bedded and ripple-bedded sandstones have the lowest values of permeability. Original depositional environment and bedding style exercise important control on permeability (particularly potential gas flow) even in rocks in which the pore systems have been modified significantly by diagenesis. A knowledge of depositional environment and bedding style is therefore important in predicting potential producibility in tight, gas sandstones such as the Travis Peak formation.

Mineralogy, Petrography and Derivation of Sands and Silts of the Continental Slope, Rise and Abyssal Plain of the Gulf of Mexico

ABSTRACT Sand and silt interbeds in cores from the continental slope, rise and abyssal plain of the Gulf of Mexico, may be composed of either detrital or carbonate sediments. Because of the insensitivity of the detrital minerals to transport distance and environment, the sand and silt interbeds from the deep portions of the Gulf may be related to specific source areas on the continental shelf. These source areas include (1) the Mississippi, (2) the Rio Grande, and (3) the rivers of northeast Mexico. Vertical variations in mineralogy show no significant trend with increasing depth in any core, indicating that relative contributions from each source remained constant. Carbonate sands and silts of the abyssal plain were derived from the shallow waters of the Campeche Shelf. Transportation along t e axis of the Campeche Canyon carried these shelf carbonates northward into deeper water areas. in some instances through the medium of turbidity currents.

Sedimentological Indices of Transport Direction, Distance, and Process Intensity in Glacio-fluvial Sediments

ABSTRACT Examination of sediments at the terminus of the Mendenhall Glacier, Juneau, Alaska, indicates that the relative volumetric abundance of various pebble types can be used to determine direction of transport from source terrane and differences in energy flux between contrasted depositional environments. Both an increase in transport distance and an increase in energy flux independent of transport distance result in a relative enrichment of mechanically, strong detrital rock species. Extremely strong and extremely weak detrital species produce smooth compositional gradients with respect to source terrane and are relatively unaffected by differences in depositional environment. Rock species of intermediate resistance to disintegration reflect both down-valley degradation and an environment l effect. The general nature of functional relationships between the relative abundance of a detrital species and distance is greatly influenced by the spectrum of resistances associated with the assemblage and the variations in degradational energy affecting the detritus.