Christopher G. St. C. Kendall

Carbonates and relative changes in sea level

Abstract In the geologic record some of the most accurate gauges of changes in sea level are the sediment type, geometry and diagenesis of carbonate shelves and platforms. This is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics. World-wide changes in relative sea level (the sum of eustatic sea-level changes, sedimentation and crustal movements) have occurred repeatedly and cyclicly through geologic time, producing characteristic responses in carbonates. 1. (1) Relative rises in sea level (usually caused by the cumulative effect of tectonic subsidence and eustatic rise) may result in the following: 1.1. (A) Drowned carbonate reefs or platforms. Here carbonate growth potential is exceeded by relative sea-level rise, and is characterized by shallow-water sediments, overlain by hardgrounds and/or deep-water sediments, some of which may be condensed sequences. 1.2. (B) Platforms where only the fast growing rim and patches of the interior are able to match sea-level rise while the remainder of the platform is drowned (temporarily). 1.3

Sequence stratigraphy: methodology and nomenclature

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

Recent algal mats of a Persian Gulf lagoon

Extensive laminated mats of algae are forming on the protected intertidal and supratidal flats of a highly saline lagoon, the Khor al Bazam, Abu Dhabi, southwest Persian Gulf. At the east end of the lagoon, the largest algal flat parallels the coast for 42 kilometers, and to the west another smaller one parallels the coast for nine kilometers. These flats, part of the seaward edge of a prograding coastal flat, have an average width of approximately two kilometers and a thickness of at least thirty centimeters. In some areas they extend landward in the subsurface for more than two kilometers, beneath a thin cover of evaporites and wind blown and storm washover sediments. Smaller flats occur in the shelter of islands, headlands, and swash bars. The larger algal flats are divided on the basis of surface morphology, into four geographical belts. From the high-water mark seaward these are: (1) Flat zone--firm, smooth algal mat, with no topographic relief, overlying quartz-rich carbonate sand and evaporites; (2) Crinkle zone--leathery algal skin forming a blistered surface over gypsum and carbonate mush; (3) Polygonal zone--algal mat separated into desiccation polygons a few centimeters to two meters in diameter, which cover laminated algal peat; carbonate sand and mud fills the cracks between the polygons. (4) Cinder zone--a warty black algal surface, the color and size of the raised bumps resembling a weathered volcanic cinder layer. These bumps, shaped like small pustules two to three centimeters in diameter, cap an unlaminated algal and sediment peat. The algal growth and structures appear to be determined by the frequency and duration of subaerial exposure and the salinity of the tidal waters; they are only related to wave energy when limited by wave and tidal scour at the edge of the Cinder zone and along ebb channels.

Comparison of Sequences Formed in Marine Sabkha (Subaerial) and Salina (Subaqueous) Settings--Modern and Ancient

Marine evaporites occurring in modern subaqueous (salina) settings and subaerial (sabkha) settings are different. Subaqueous Holocene evaporites occur as shoaling-upward lacustrine sequences up to 10 m thick. They are evaporite dominated and are composed primarily of bottom-nucleated crystals that may be deposited as massive, laminated, or rippled units. Each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments. In plan view, they show a well-developed bulls-eye pattern with a sulfate center and a carbonate rim. In contrast, subaerial (sabkha) evaporites occur as part of a laterally prograding, shoaling-upward, peritidal sequence in which the supratidal unit is usually no more than 1 m thick. Sabkha sequences are matrix dominated, not ev porite dominated, with the bulk of the sulfate phase occurring as diagenetic nodules, enteroliths, or diapirlike structures. These sulfates were formed during syndepositional diagenesis by replacement and displacement processes. The various facies of the sequence tend to accumulate in belts parallel with the shoreline. Relative to the sea level or the brine level, sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows. Some of the characteristics that distinguish Holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies, even when gypsum has been converted to nodular anhydrite. The distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates.

Out of our depth: on the impossibility of fathoming eustasy from the stratigraphic record

The evolution of a sedimentary basins gross morphology depends upon (1) basement movement, (2) sediment accumulation and compaction, and (3) variations of eustasy with time. Thus if an accurate history of eustasy could be derived, it would aid in our understanding of basin stratigraphy and help in the prediction of sediment geometry from described areas. Techniques which attempt to determine the magnitude of eustatic sea-level excursions include the measurement of (1) the amount of sedimentary onlap onto the continental margins with and without the use of hypsometric curves, (2) the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines, (3) the perturbations on individual thermo-tectonic subsidence curves and stacked crustal subsidence curves, (4) the variations in deep-ocean oxygen isotopes found in sediments, and (5) the size of variables used in graphical and numerical simulations of basin fill in terms of tectonic behavior, rates of sediment accumulation and eustasy which “invert” the problem. To date a combination of the use of relative sea-level charts derived from sediment onlap of the continental margin with dimensioning by oxygen isotopes responding to glacial events offers the best potential for relative (tectono/eustatic) sea-level curves, but even this method can not produce a unique solution for absolute eustatic variations. Mathematical modelling shows that, at best, it is possible to obtain only the sum of tectonic basement subsidence and sea-level variations from the above methods, and, at worst, not even that simple a combination. Thus, every proposed scheme to measure eustatic sea-level excursions assumes some behavior for two of the three underlying processes: tectonic movement of the basement, sedimentary accumulation and eustatic sea level. Each scheme then determines the third process relative to the assumed model behavior of the other two. It would seem that, like King Canute, we cannot command the sea though we can still use undifferentiated “relative” (tectono/eustatic) sea-level curves to generate a “family” of solutions which are the product of a variety of absolute sea-level curves and tectonic models. Each solution has then to be assessed in terms of geologic setting and the hypotheses it generates.

Holocene Shallow-Water Carbonate and Evaporite Sediments of Khor al Bazam, Abu Dhabi, Southwest Persian Gulf

Holocene shallow-water carbonate and evaporite sediments are forming in the Khor al Bazam, a saline lagoon on the coast of Abu Dhabi, southwest Persian Gulf. The sediments are composed of (1) skeletal grains, including whole and fragmented mollusks, corals, calcareous algae, bryozoans, foraminifers, and ostracods; (2) nonskeletal grains, including oolites, pellets, and pellet aggregates; (3) carbonate mud; (4) noncarbonate minerals including gypsum, anhydrite, and terrigenous quartz; and (5) organic material incorporated within mud and carbonate grains. Lithofacies are (1) coral and coralline-algae facies, (2) oolitic sand facies, (3) pellet-aggregate and pellet facies, (4) mud and pellet facies, (5) molluscan sand facies, (6) algal mat facies, and (7) evaporite facies. Nonskeletal calcium carbonate sediment types are related to wave energy; oolites form in the most turbulent environments, pellet aggregates in moderately sheltered environments, and pellets and muds in areas of low wave energy. Skeletal material is altered by blue-green algae. The grains contain algal cells and threads invested in mucilage. Respiration and photosynthesis by algae dissolve the original shell material and reprecipitate microcrystalline aragonite. Chemical changes within the investing organic material cause blackening of some of the carbonate grains.

Upper Permian (Guadalupian) Facies and Their Association with Hydrocarbons--Permian Basin, West Texas and New Mexico

Outcrops of Guadalupian sedimentary rocks in the Permian basin of west Texas and southeastern New Mexico are a classic example of the facies relationships that span a carbonate shelf. In the subsurface, these rocks form classic hydrocarbon-facies traps. Proceeding from basin to the updip termination of the shelf, the facies are (1) deep-water basin, (2) an apron of allochthonous carbonates, (3) carbonate shelf margin or reef, (4) carbonate sand flats, (5) carbonate barrier islands, (6) lagoon, and (7) coastal playas and supratidal salt flats (sabkhas). Over a half century of exploration drilling has shown that hydrocarbons in the Permian rocks of the Permian basin have accumulated at the updip contact of the lagoonal dolomites and clastics with the coastal evaporites, and in the basinal channel-fill clastics. The shelf marginal (reef) facies contain cavernous porosity, but commonly are water saturated. These facies relationships and hydrocarbon occurrences provide an exploration model with which to explore and rank hydrocarbon potential in other carbonate provinces.

Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays

Abstract Consolidated to friable carbonate rocks found in the Lee Stocking Island area in the Exuma Cays include: (1) reef rock, (2) channel stromatolites, (3) shallow-water hardgrounds, (4) beachrock rimming the islands and (5) Pleistocene bedrock. The most common cement fabrics observed are: aragonitic fibers, which include acicular fan-druse and square-tipped coarse fibers cementing beachrock and stromatolites; and an isopachous needle-fiber rim cementing hardgrounds and stromatolites. Less common are high-Mg calcite bladed textures of the reef rock and stromatolites. Two types of blades are present: the more common stubby variety of either high-Mg or low-Mg calcite, and an elongated variety of high-Mg calcite which was found in only three beachrock samples. Aragonitic micrite envelopes usually surround grains in beachrock, hardgrounds and stromatolites, but only in association with fibrous cement. An aragonitic crust cements the surfaces of lime mud beds of the tidal channel, while a high-Mg calcite cryptocrystalline cement occurs in all the rock types. Calcified algal filaments of high-Mg calcite, from the abundant green and blue-green algae in the area, are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock. A low-Mg calcite equant spar cements the Pleistocene samples and is associated with meteoric diagenesis and cementation of the Pleistocene surface. Cement precipitation coincides with the path of the cool oceanic water from Exuma Sound as it warms and loses CO 2 and moves up onto the bank near Lee Stocking with the incoming tide. Cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward, fibrous cements become predominant. As suggested by their crystal size and location on the shelf margin, we think that the reef rock cryptocrystalline material are the fastest forming of the cements, where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming. The rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate. On the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds, but these may form at a slower rate than that of the margin, though this rate is still quite high. Cementation gradients occur from the tidal channel to the intertidal zones of: (1) west Normans Pond Cay, where cement fabric suggests a reduced calcium carbonate availability, and (2) west Lee Stocking Island, where a change in mineralogy suggests a change in water chemistry. Thus, a sequence of cement fabrics and mineralogies can be traced. Micritic textures occur in a more seaward position; fine, fibrous aragonite fibers in a more lagoonal and levee position; and coarser aragonite fibers and Mg-calcite cements in the intertidal and supratidal position. This sequence is thought to track the evolution of the water mass.

Depositional Setting of the Upper Jurassic Hith Anhydrite of the Arabian Gulf: An Analog to Holocene Evaporites of the United Arab Emirates and Lake MacLeod of Western Australia

The Upper Jurassic Hith Anhydrite is a major hydrocarbon seal in the Arabian Gulf region. Outcrops, core samples from the subsurface, and the literature indicate that the Hith Formation is composed mainly of anhydrite. In most locations where a section of the Hith Formation has been measured, this unit contains less than 20% carbonate, much of which is in the form of thin laminations. This lack of carbonate, locally thick layers of salt, and the predominance of anhydrite favor a playa for the setting in which this sediment was accumulated. In fact, much of the Hith has the sedimentary characteristics of the Holocene Lake MacLeod playa of Western Australia, which is dominated by layers of gypsum and halite (what little carbonate that occurs is found in layers at the base o the section). Locally the Hith appears to have accumulated in a sabkha setting, particularly toward central Abu Dhabi where it pinches out into shallow-water, and peritidal carbonate. This sabkha setting is indicated by the interbedded relationship of the Hith anhydrites with these carbonates and the local predominance of horizontally flattened nodules and enterolithic layers of anhydrite. These latter features match some of the characteristic fabrics found in the Holocene coastal sabkhas of the United Arab Emirates. As with the local occurrences in the Hith, the Holocene sabkhas are dominated by carbonates and are divisible into a series of lateral facies belts. These are also expressed as equivalent vertical layers. Traced from seaward to landward, or from the base of the vertical sequence upward these facies are characterized by (1) algal mat, (2) a layer of a gypsum crystal mush (3) active anhydrite replacement of gypsum (4) anhydrite with no gypsum mush, and (5) recycled eolianite and storm-washover sediments.

Permian-Triassic boundary in eastern Nevada and west-central Utah

The Permian-Triassic boundary in eastern Nevada and west-central Utah is placed at a disconformity above the Gerster Formation (Wordian) and below the Thaynes Formation (Smithian and Spathian). Evidence of subaerial erosion at the disconformity includes local truncation of Gerster beds by channels filled with chert and quartzite-pebble conglomerate. Westward onlap of Lower Triassic sedimentary deposits, as demonstrated by conodont zones, suggests an east-facing paleoslope over which the Early Triassic sea slowly advanced. The Sonoma orogeny, which coincided with the Late Permian—Early Triassic hiatus, may have been the cause of regional upwarping in eastern Nevada that produced the eastward paleoslope.