Richard Rezak

Porometry and fabric of marine clay and carbonate sediments: Determinants of permeability

Abstract The porometry of a marine sediment is determined by the fabric, i.e., the shape, orientation, arrangement (spatial distribution) and associations of the solid particles. Fabric ultimately determines the permeability of the sediment by controlling the size, shape and continuity of the pore space. Electron microscopy studies have revealed complex fabrics and porometries of surficial and deeply buried sediments from various geological environments. When viewed in profile using thin sections, the pores of unconsolidated, high-porosity marine clays are often very irregular in shape and span a broad size range. In general, however, the pore profiles of these sediments can be described as having aspect ratios (length-to-width ratios) which approach 1.0. Pore profiles of consolidated, low-porosity clays are characterized by aspect ratios which approach infinity. Coefficients of permeability for marine clays typically range from 10 −5 cm/s (porosities of 70–75%) to 10 −8 cm/s (porosities of about 50%). The permeability coefficient was 10 −13 cm/s and the porosity was 27% for bentonite (a smectite) consolidated in the laboratory to a maximum load of 68.9 × 10 3 kPa ∗ . The pore space characteristics in carbonate sediments vary greatly depending on the constituent particles. Aspect ratios for pore profiles can range from 1.0 in a coccolith ooze to 2.0 in an aragonitic needle matrix consolidated uniaxially at pressures of 4.32 × 10 3 kPa. The permeability of shallow-water (3–4 m) oolitic carbonates is largely controlled by the permeability of the aragonite needle matrix. Despite the large quantity (in one case 94% by weight) of fine-sand-size ooids in these deposits, the comparatively impermeable ooids are matrix supported. This results in a sediment with relatively high porosity and medium permeability, similar to fine sands that are tightly packed and, therefore, grain supported. Permeability coefficients of these oolitic carbonate sediments range from 10 −2 to 10 −4 cm/s. Carbonate sediments recovered from intermediate water depths (970–1980 m) are composed of grains (mostly large shell fragments) embedded in either a coccolith matrix or an aragonite needle matrix. The permeability coefficients for these carbonates range from 10 −5 to 10 −6 cm/s. The sediments studied range in size from sands to clays, and the differences in porosity and permeability observed for these sediments reflect differences both in the fabrics of the fine-grained matrices and in the grain-size distributions. The data presented suggest that the sediment fabric is a function of the characteristics of the constituent particles and the physical and chemical environments of deposition and that the fabric plays a major role in determining the permeability of the deposit. Limited but revealing computerized image analyses have been carried out on scanning and transmission electron micrographs of these sediments. Initial results indicate that this approach may prove useful for quantifying fabric parameters and providing a statistical basis for fabric descriptions.

Evolution of Yucca Mound Complex, Late Pennsylvanian Phylloid-Algal Buildup, Sacramento Mountains, New Mexico

The Late Pennsylvanian Yucca Mound complex is a carbonate buildup on the western side of the Sacramento shelf facing the Oro Grande basin, in south-central New Mexico. This complex is an accumulation of algal and foraminiferal boundstones and phylloid-algal packstones and wackestones which have been subjected to several periods of subaerial exposure. The growth history of this organic complex is related closely to late Paleozoic cyclic sedimentation in a tectonically active area, and is thought to be the result of progressive offlapping growth stages down the western flank of the active La Luz anticline. The oldest beds exposed in the complex (plumose masses of algal and foraminiferal boundstone) formed within wave base on the crest of an earlier organic buildup. The crest of the older buildup was subaerially exposed while a thick accumulation of carbonate mud replete with the phylloid algae, Ivanovia, started forming against its western (seaward) flank. As sea level rose and submerged the earlier mound, the phylloid-algal accumulation kept pace with sea-level rise and onlapped the older mound, despite a few brief interruptions. When the phylloid algae built up into wave base, and attained its climax stage of evolution, a typical shallow-water facies developed. This distinctive rock type on the preserved, rounded mound top contains abundant forams, both encrusting and mobile chambered forms, together with fish remains and conodonts. A bedded detrital flank facies, composed of broken phylloid-algal material, and concentrated remains of forams, fish, and conodonts that inhabited the upper mound, is present on the shelfward (lee) side of the complex. Abundant, algally coated skeletal grains characterize the flank facies, which is easily recognizable elsewhere within the area, mainly in mound-flanking beds. When sea level dropped, Yucca Mound growth ceased, and smaller boundstone accumulations were formed down the offshore flank of the main mound complex. The varied growth history of this particular organic buildup demonstrates just how much opportunity existed for subaerial and littoral diagenetic processes to act on and to modify the varied rock facies common in late Paleozoic phylloid-algal buildups.

Deep-Sea Carbonates

Calcium carbonate in sediments can greatly effect the engineering properties of those sediments. Factors that may effect engineering properties are: 1) great variation in primary void ratios, (2) inherent strength of carbonate grains, (3) degree of sorting, (4) ratio between grains and matrix, (5) nature and amount of cement, and (6) depth of burial. Deep-sea carbonates appear to be anomalous but do exist in rather large quantities. Their existence must depend upon a combination of physico-chemical and organic-chemical factors that need to be investigated in much more detail.

Seafloor instability at East Flower Garden Bank, northwest Gulf of Mexico

An increasing volume of evidence suggests that normal faulting and graben formation associated with salt diapirism may be catastrophic. The evidence includes: 1) removal of solid salt from the diapirs by dissolution, 2) the nature of rock outcrops at the crests of domes, 3) seismic profiles showing recent displacement of the seafloor, and 4) changes in coral growth rates at the East Flower Garden Bank. The possibility of catastrophic movement of the seabed is an engineering constraint that must be addressed prior to emplacing structures such as jack-up rigs and production platforms.

Mississippi-Alabama Outer Continental Shelf Topographic Features Formed during the Late Pleistocene-Holocene Transgression

A geophysical survey of the outer Missisippi-Alabama continental shelf revealed three types of topographic features: 1) reef-like mounds (RLM), 2) isobath-parallel ridges, and 3) shallow depressions. RLM appear to be bioherms. Their bases cluster in two isobath bands, 105 to 120 m and 74 to 82 m, implying the late Pleistocene-Holocene transgression was nonuniform. The deeper RLM probably formed during a slow sea level rise at the beginning of the deglaciation. An 11 ka BP C14 age was obtained from the summit of one, at a depth of 90 m. Depth relationships suggest the shallower RLM and most ridges formed about the time of the mid-deglacial Younger Dryas cooling episode.

A tale of two deltas: seismic mapping of near surface sediments on the Mississippi–Alabama outer shelf and implications for recent sea level fluctuations

Abstract High-frequency seismic reflection profiles showing near-surface sediments of the Mississippi–Alabama outer shelf define two shelf-margin deltas and three unconformities. The deepest unconformity lies below the westernmost delta, the middle unconformity separates the two deltas, and the shallowest unconformity is atop the easternmost delta. The deeper two unconformities are extensive and erosional to the shelf edge, implying Type-1 sequence boundaries. The shallowest is restricted in its depth range and extent, suggesting a Type-2 boundary. Both delta wedges contain sigmoid reflectors, representing foreset beds, and have apparent transgressive layers at their bases. Thus, these two sediment wedges and their bounding unconformities represent parts of three sea level cycles. Although we lack good age constraints, the middle unconformity appears to represent the last glacial maximum, so the western delta must have been deposited before and the eastern delta, afterwards. Internal reflector geometries suggest the eastern delta was deposited during an episode of falling sea level, implying a relative sea level regression within the overall deglacial transgression.

Basalt from Louisiana continental shelf

A basalt outcrop was discovered on Alderdice Bank on the outer Louisiana continental shelf. The basalt shows an age of 76.8 ± 3.3 × 106 years. Textural, mineralogical, and chemical characteristics indicate that it is an alkali basalt of shallow intrusive origin. It was probably brought to the seafloor by salt tectonics and exposed due to salt dissolution.An accurate account of the Mesozoic geologic history of the Gulf of Mexico must consider the apparent consanguinity of all magmatic rocks of the region, including the Alderdice Bank basalt, and the apparent basinward decrease in age of magmatic activities.

Consolidation-related fabric changes of periplatform sediments

Geotechnical properties of carbonate sediments result from several factors such as the particular constituents comprising the sediment, the mineralogy, fabric, and effective stress. Investigating the effects of increasing effective stress by mechanical consolidation using permeability, porosity, and porometry determinations reveals fabric changes that could not be determined in scanning electron micrographs. Porometry changes, determined using an image analyzer, are a function of matrix composition and grain support. Samples that were predominantly matrix-supported exhibited an increase in pore numbers per unit area and a pore size decrease resulting from consolidation. Samples composed predominantly of aragonite needle matrices displayed opposite behavior.

Chapter 6 Reconnaissance of Reefs and Fishing Banks of the Texas Continental Shelf

Publisher Summary This chapter discusses the investigation study of reefs and fishing banks of the Texas continental shelf. Biotic assemblages of reefs and hard-banks of the Texas–Louisiana outer continental shelf can be distinctly grouped according to their natures into four categories: (1) the sparse Claypile Bank biota (35–55 m) of predominantly low-growing filamentous and leafy algae and sponges with occasional “meadows” of high-standing leafy algae occupied by numerous fish, (2) the more diverse Stetson bank and Three Hickey rock biota (28–56 m) dominated by the hydrozoan fire coral Millepol alcicornis and sponges, (3) the highly diverse and abundant flower gardens/twenty-eight Fathom bank biota with coral reefs (22–49 m), algal nodule and sand-covered platforms (45–76 m), and drowned reefs from 76 to 100+ m, bearing an assemblage of organisms directly comparable to the deep-water biota of category 3, and (4) the deep-water biota of the South Texas fishing banks (56–78 m) and fishnet bank (61–82 m) are characterized by the presence of antipatharian whips, deep-water alcyonarian fans, comatulid crinoids, certain species of deep-dwelling fishes, and sparse populations of encrusting coralline algae. This chapter discusses about the coral reef at West Flower Garden Bank. It also presents some findings on antipatharian zone.

Chapter 10 Environmental Effects of a Natural Brine Seep at the East Flower Garden Bank, Northwestern Gulf of Mexico

Publisher Summary This chapter describes anoxic sulfide-rich brine of nearly ambient temperature, which percolates from the sea floor, forming a small, shallow lake of dense water in a depression at the eastern margin of the East Flower Garden (EFG) bank. Residence time for the brine in the lake is less than one day. Brine overflows the lake and is substantially mixed with seawater in the axis of a 60 m long canyon extending from the lake to the bank edge, where dilutions are greater. The chemical composition of the brine indicates that it is a product of dissolution of the salt by seawater percolating through cracks, faults or permeable strata. Sulfide-oxidizing bacteria are abundant at the brine seawater interface and on the canyon floor where mixing brine and entrained seawater flow in a recognizable stream along the bottom. The EFG seep provides an example of a possible management technique for brine discharges.