Conrad D. Gebelein

The Composition, Structure and Erodability of Subtidal Mats, Abaco, Bahamas

ABSTRACT The composition and microstructure of widespread subtidal biological mats binding sandy carbonate sediments in the Rock Harbour Cays vicinity of Little Bahama Bank were examined in detail; these mats were subjected to in situ flume experiments. The mats consist of various assemblages of green algae, red algae, blue-green algae, diatoms and animal-built sand grain tubes. Green algae, red algae and/or sand grain tubes provide a rigid open network into which grains infiltrate and are trapped. The mucilaginous secretions of both blue-green algae and diatoms in association with the fine filaments of blue-green algae bind the grains to each other and to the mat network. On the basis of composition and microstructure, three basic mat types were recognized: a fibrous, rigid, Cladoph ropsis mat; a thin, gelatinous, Lyngbya mat; a cohesive, aggregated, Schizothrix mat. The erosion by artificial currents of initially undisturbed mats was studied in the field using an underwater flume, and the complex manner in which the mats disintegrated was recorded. The surface sediment from each mat area was then treated with NaOCl to remove the organic matter and erosion tests repeated in a tank in the laboratory with the same apparatus. The natural, in situ, mat-bound sediment could withstand current velocities at least twice as high and, in some cases, as much as five times as high as those that eroded the treated, unbound sediment. The intact mat surface could withstand direct current velocities three to nine times as high as the maximum tidal currents 13 cm/sec) recorded in the mat environment. Each mat type eroded in a characteristic manner and sequence dependent upon the mat composition and microstructure. This breakdown process differed markedly from the erosion behavior of loose sediment. Observations indicate that grain size, sorting, packing, structure and sediment surface morphology are influenced by mat formation. This study demonstrates the need for close consideration of interfacial biological communities when examining depositional and erosional processes at the sediment-water interface, or when making interpretations from the products of these processes in ancient rocks.

Algal origin of dolomite laminations in stromatolitic limestone

ABSTRACT Many ancient carbonate rocks consist of dolomite interlayered with limestone on a centimeter or millimeter scale. Many such rocks contain cryptalgal laminites or stromatolites. Comparable Recent stromatolitic and flat laminated algal sediments are composed of alternating layers of particulate carbonate sediment and algal mats. The characteristics of algal-rich and sediment-rich laminae are identical to those of the dolomite and calcite laminae, respectively, of ancient forms. On this basis, dolomite laminae in ancient interlaminated sediments are considered equivalent to, and hence derived from, algal-rich lamina in Recent stromatolitic sediments; calcite laminae are derived from sediment-rich laminae. Modern algal sediments of this type, however, are not known to contain thin layers f dolomite. It is suggested that the dolomite layers are secondary. During deposition, magnesium ions are complexed organically in the algal mat layers. The algal sheath material, in which the magnesium is concentrated, is very stable and does not decompose until long after deposition, or even lithification. Only when the organic matter is decomposed is the magnesium released to form dolomite in the micro-environment of the relict algal mat layers. Thus the dolomite, although secondary, conforms to the primary algal mat layers of the sediment. Laboratory experiments support the hypothesis. Sheath material of the major stromatolite-forming blue-green alga Schizothrix calcicola grown in sea water shows a three to four fold increase in Mg/Ca ratio relative to that in the sea water medium. Sufficient magnesium is complexed in a single 2 mm thick algal mat layer to produce a layer of dolomite 1 mm thick. No dolomite was precipitated in the algal mat layers experimentally, but crystallization of 17-20 mole percent-Mg calcite in contact with the sheath material was achieved, showing the influence of the organic layer micro-environment on the composition of the carbonate mineral produced. The actual formation of dolomite may be a long term process, probably achieved by the partial replacement of the carbonate matrix in conta t with the relict algal mat layers, and/or by selective replacement of high-Mg calcite within the algal mat layers. The hypothesis explains why thinly interlayered dolomite is not found abundantly in Recent sediments, and yet why the dolomite layers so faithfully conform to the primary sedimentary structures in ancient rocks. The hypothesis refers specifically to laminated cryptalgal rocks, but should be considered for other problematical types of ancient dolomite where the distribution of dolomite may be related to the original distribution of organic matter in the sediment.

Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia

Following on the research presented in AAPG Memoir 13, which focused on environment and Quaternary history of Shark Bay, this publication examines the same area again, but with a strong stratigraphic emphasis running as a common thread through all 7 papers in this volume.

Mixing Zone Dolomite in Tidal-Flat Sediments of Central-West Andros Island, Bahamas: ABSTRACT

The central-west coast of Andros Island is a complex carbonate facies mosaic, deposited at or near mean sea level. The shore has prograded intermittently during the past 3,000 years to produce a seaward-thickening wedge of sediments up to 25 km wide and about 4 m thick beneath the present shoreline. Old channel levees and beach ridges have been preserved during progradation as topographically high ridges which allow the development of freshwater lenses beneath. These lenses show considerable seasonal variation, both in geometry and pore-water chemistry. Mixing zones, extending laterally a few hundred meters around the lenses and down to Pleistocene bedrock, separate the fresh waters of the lenses (with high alkalinity and low ionic concentration), from adjacent saline and hypersaline waters (with low alkalinity and high ionic concentration). Fresh waters are generally undersaturated with aragonite, calcite, and dolomite, but calcite saturation may occur locally. The ends of aragonite needles, skeletal grains, and pellets are commonly corroded, although low-magnesian calcite crystals may locally enclose and replace aragonite by dissolution-reprecipitation. Water in the mixing zone, between 2,500 and 15,000 ppm Cl-, is undersaturated with respect to aragonite and low-magnesian calcite, but supersaturated with respect to calcium magnesium carbonates. X-ray diffraction (XRD) studies indicate small amounts of (< 6%) of protodolomite (38 to 44 mole % of MgCO3) distributed in patches across whichever sedimentary facies are intersected by the mixing zone. The mixing-zone origin of the protodolomite is however equivocal, as similar compositions occur more rarely with saline pore waters not associated with present or past mixing zones. SEM reveals 1µm euhedral rhombic protodolomite crystals between and engulfing aragonite needles. The needles may later dissolve. Chemical data from mixing zones indicate precipitation of a magnesium-rich phase frequently in excess of observed protodolomite concentrations; a huntite phase is indicated by considerations of stability, but is unsupported by XRD results. End_of_Article - Last_Page 457------------

Algal Mats, Cryptalgal Fabrics, and Structures, Hamelin Pool, Western Australia: ABSTRACT

Cryptalgal sediments and structures in Hamelin Pool, Western Australia, are formed by interactions between blue-green algae, which trap and bind sediment particles, and a variety of mechanical and diagenetic processes. The algae form a cohesive mat that tends to cover intertidal, supratidal, and some shallow subtidal surfaces. The mat is differentiated into seven intergradational types as an expression of variations in algal species present, ratio of filamentous to unicellular forms, quantity of mucilaginous matrix, life habits, and quantity and nature of the host sediment. The distribution of mat types is controlled by environmental factors such as elevation of substrate, drainage, depth and nature of interstitial groundwater, and sediment influx. In tidal flats with gen le gradients, there is a broad zonation of mat types, whereas the mat is highly differentiated and has a condensed, patchy development of types on headlands and locations with irregular topography. The sediments trapped and/or bound by the algal-mat communities are imprinted with distinctive fabrics. These fabrics, which can be related to specific mat types, reflect a complex interaction between the algae and processes of sedimentation and diagenesis. Important factors in the development of fabric are surface texture and internal structure of the mat, rate and frequency of sediment influx, and processes such as oxidation, cementation, and lithification. Changes in mat type with changes in environmental conditions (e.g., shoaling and sediment influx) lead to the development of successions of fabrics in the sediment pile. The mat-sediment complex is shaped by physical factors into a variety of structures: (1) extensive flat-lying sheets, (2) ridge and rill structure, (3) rings and crescents, (4) linked ellipsoids and columns, (5) discrete ellipsoidal and circular columns, and (6) calyx and tiered calyx structures. The size range of structures varies from a few centimeters to several meters; confluent and branched structures also are common. The gross morphology of the structures is largely independent of the mat type (or types) involved in the primary trapping and/or binding processes. Major environmental factors involved in shaping of structures are waves, currents, substrate gradient, and long-term sea-level change; minor factors include burial, exhumation, growth of epiphytes, activity of browsing organisms, gas evolution, corrosion, precipitation, desiccation, and variation in sediment type. These factors also influence the external surface texture of structures.

Composition, Structure, and Erodability of Subtidal Mats, Abaco, Bahamas: ABSTRACT

In the Rock Harbour Cays, near Little Bahama Bank, the composition and microstructure of widespread, subtidal, sediment-binding, mat communities were examined and described. Mat-bound and unbound sediment surfaces were then subjected in situ to flume-created direct currents to test both resistance to erosion and breakdown behavior. The mat was removed by bleach treatment and the flume tests were repeated. The mats consist of algae, diatoms, and arenaceous tubes. Algae and/or tubes provide the resistant framework within which grains accumulate and are bound by mucilaginous secretions plus fine algal filaments. Three mat types were distinguished: a fibrous Cladophoropsis mat, a gelatinous Lyngbya mat, and an aggregated Schizothix mat. Each mat type eroded in a characteristi manner and sequence dependent on mat composition and microstructure. Areas of intact mat withstood erosion better than irregular or broken surfaces. Mat-bound sediment surfaces withstood current velocities of more than 100 cm/sec, more than five times those required to erode mat-free surfaces. These studies indicate that particle size, sorting, packing, structure, and bedding-plane morphology are influenced by the presence of mats. Thus, consideration of mat communities is important when examining depositional and erosional processes at the sediment-water interface, or when making interpretation from ancient rocks which are the products of these processes. End_of_Article - Last_Page 734------------

Sedimentology and Stratigraphy of Recent Shallow-Marine and Tidal-Flat Sediments, Southwest Andros Island, Bahamas: ABSTRACT

Shallow-marine and tidal-flat carbonate sediments form a seaward-prograding wedge (maximum 6 m thick) on the southwest coast of Andros Island. Sediments have been deposited in a large (85 × 25 km), arcuate embayment in Pleistocene limestone. Incursion of the sea over freshwater peat reached the inner, central margin of the embayment 5,000-7,000 years ago. Subsequent lateral progradation (maximum 30 km perpendicular to strike) of marine and tidal-flat sediments has occurred in several phases. 1. Continuous lateral progradation of a very shallow (less than 1 m), wide (3-5 km) subtidal platform adjacent to the shoreline. Sediments on the platform are poorly sorted, massive, fossiliferous, white-pellet muds with prominent filled burrows. 2. Development of 5 major shoreli es, roughly parallel and 2-5 km apart, during the past 5,000 years. Each new shoreline developed, presumably by storm action, onto the subtidal platform, and isolated a shallow linear lagoon. Abandoned shorelines form parallel bands of discrete to coalesced, End_Page 780------------------------------ symmetrical, V-shaped hummocks. These vegetated ridges (up to 1 m high) are positions of former beach ridges and tidal-channel levees. Sediments in the hummocks are thin-bedded (3-15 cm), unlithified pellet sands with pronounced fenestral fabrics. Sloping flanks of the hummocks are composed of lithified crusts (up to 3 cm thick), commonly separated by unlithified sediment layers. 3. Initial infilling of the lagoons in the form of closely spaced, circular to ellipsoidal Carolina bays. Bay margins form by spit accretion of well-sorted pellet and skeletal sands. The bays themselves fill with poorly sorted, muddy pellet sands. 4. Bay sedimentation generates a mosaic of isolated small sand bodies within a muddy, pelletal, massive sediment. Vertical infilling of the lagoons in the form of l terally continuous, alternate thin beds of blue-green algae and pellet mud. (5) Capping of the sequence by laterally continuous, thin crusts of aragonite-dolomite. End_of_Article - Last_Page 781------------

Sedimentology and Ecology of Holocene Carbonate Facies Mosaic, Cape Sable, Florida: ABSTRACT

The Holocene carbonate sediments of Cape Sable, Florida, form a facies mosaic in which facies are controlled by frequency and duration of flooding. The 4 following zones occur: 1. Flooding 0-5% of the time (supratidal)--massive to crudely bedded sandstone or siltstone, abundant birdseye, low species diversity, high abundance of single species with uniform-sized individuals. 2. Flooding 5-25% of the time (high intertidal)--low domal and flat laminated algal stromatolites, desiccation cracks and flat laminated pebbles, low species diversity, low abundance of individuals, microscopic invertebrates only. 3. Flooding 25-90% of the time (low intertidal)--massive End_Page 339------------------------------ burrow-mottled silts, moderate species diversity, moderate abundance of individuals, mainly microscopic, with a wide size range among individuals. 4. Flooding > 90% of the time (subtidal)--massive pelletal silts and clays, highly burrow-mottled, high species diversity, high abundance and wide size range of individuals within species, many macroscopic invertebrates. Areas of high intertidal and supratidal sediments where ponding of waters occurs for extended periods are characterized by single or multiple algal and sediment laminae much thicker than in areas where waters drain rapidly. Sedimentation in zone 1 forms thin beds, derived from sediment-laden waters driven over the area during storms. In Zone 2, sediments are deposited in thin laminae; sedimentation is controlled by the trapping of particles carried by tidal currents and binding them onto mats of blue-green algae. Sedimentation in zone 3 occurs mainly in the form of thin beds deposited during storms and subsequently reworked by organisms. In zone 4, deposition occurs by settling of (1) in situ sediments; (2) particles carried into the area by tidal currents; and (3) particles from sediment-laden storm waters. Measurement of production of calcareous sediment within the Cape Sable area, measurements of the net transport of sediment into the area by tidal currents, and measurement of the volume of sediment deposited in the area since its opening to the sea in the 1920s allow the following calculations to be made. Since 1920, 4% (0.01 cm/yr) of the total deposit has been derived from in situ production, 34% (0.28 cm/yr) by net transport into the area on tides, and 62% (0.50 cm/yr) by storms. End_of_Article - Last_Page 340------------