Randolph P. Steinen

Giant subtidal stromatolites forming in normal salinity waters

We report here the discovery of giant lithified subtidal columnar stromatolites (>2 m high) growing in 7–8 m of clear oceanic water in current-swept channels between the Exuma Islands on the eastern Bahama Bank. They grow by trapping ooid and pelletal carbonate sand and synsedimentary precipitation of carbonate cement within a field of giant megaripples. The discovery is important to geologists and biologists because similar organo-sedimentary structures built by a combination of cementation and the trapping of sediment by microbes were the dominant fossil types during the Precambrian. Stromatolites are thought to have been responsible for the production of free oxygen and thus the evolution of animal life1,2. Until the discovery of small lithified subtidal columnar stromatolites in the Bahamas3, the only subtidal marine examples known to be living while undergoing lithification were in the hypersaline waters of Hamelin Pool at Shark Bay, Western Australia4–7. Shark Bay stromatolites range from intertidal to the shallow subtidal with the larger columns reaching 1 m in height. The Shark Bay stromatolites have strongly influenced geological interpretation; by analogy, many ancient stromatolites have been considered to have grown in intertidal and/or hypersaline conditions8, although hypersalinity was not a necessity for growth during the Precambrian because grazing metazoan life had not then evolved.

Whitings, a sedimentologic dilemma

Whitings, drifting clouds of water, milky because of suspended carbonate, have been claimed to originate from either the action of bottom-feeding fish or direct precipitation of calcium carbonate. Five cruises during different seasons were made to the Great Bahama Bank to collect data pertinent to the controversy. Measurements of particulate concentrations average 10 mg/liter with a maximum of 20 mg/liter of carbonate sediment suspended in whiting water, compared with an average of 1.5 mg/liter for clear water outside the whitings. The particles are dominantly acicular aragonite, but Mg calcite composes as much as 20 percent of some whitings. Sedimentation rates, measured with fixed and drifting sediment traps, were as great as 34 g/m 2 /hr. Sediment suspended in whitings aggregated into silt- and sand-size fioccules and settled to the bottom of settling tanks within six hours, even on a rocking ship. Sediment in artificial whitings, created by stirring sediment from the bottom with a shrimp trawl, settled to the bottom in about the same time. Natural whitings, on the other hand, were never observed to dissipate. Because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours, we conclude that the natural whitings are continually replenished by direct precipitation. The search for fish in whitings utilized sidescan sonar and fathometer imaging, shrimp trawls, rotenone, remote video, and direct scuba observation. These methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most Bahamian whitings. Several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend. The distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish. Stable carbon- and oxygen-isotopic analyses and Delta 14 C activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension. Estimates indicate that the amount of new carbonate produced in whitings on the Great Bahama Bank is substantially higher than that arising from algal production. Consequently, the amount of sediment transported to deep water may be much greater than previously thought.

Subaerial crusts, caliche profiles, and breccia horizons: Comparison of some Holocene and Mississippian exposure surfaces, Barbados and Kentucky

Roadcuts along Interstate Highway 64 in northeastern Kentucky expose a succession of Mississippian carbonates that contain a number of ancient exposure surfaces. These subaerial surfaces vary from thin surficial crusts to thick caliche profiles and complex brecciated horizons, and to a large extent they are similar, in both fabric and inferred origin, to subaerial surfaces developed on Pleistocene reef sediments exposed on Barbados. The most characteristic fabrics include laminations, micritic pellets, multiple episodes of fracturing, root voids, and a diversity of carbonate cement morphologies. In fact, variations in the form and distribution of the cements provide some of the most positive criteria for recognizing near-surface subaerial diagenesis. Evidence of vadose diagenesis 2 to 3 m below some of the ancient exposure surfaces indicates that relative fluctuations in sea level of at least this magnitude occurred during Mississippian time.

Phreatic and Vadose Diagenetic Modification of Pleistocene Limestone: Petrographic Observations from Subsurface of Barbados, West Indies

Data from a shallow cored borehole through 105,000-year-old reef-tract sediments on the south coast of Barbados indicate that the carbonate sediments have been altered more rapidly and more extensively in freshwater phreatic diagenetic environments than in the vadose or marine phreatic diagenetic environments. The upper part of the borehole section has been exposed only to subaerial and shallow vadose processes since emergence from the marine depositional environment about 105,000 years ago. These sediments are recrystallized only partly. Total amounts of cement, mainly needle-fiber cement and dense micrite coatings, are high. Porosity is low. Dissolution, a minor feature in this part of the section, is associated with localized development of needle-fiber fabrics. Those parts of the borehole section which have been occupied by a freshwater phreatic lens have been cemented by calcite microspar. Aragonite grains have been dissolved on a massive scale, resulting in a very well-developed moldic porosity. Neomorphic grain alteration has been volumetrically important. Packstones and grainstones in the lowest part of the borehole are largely unaltered; metastable carbonate mineralogy dominates. This part of the sedimentary column has been subjected alternately to vadose conditions during low stands of sea level (> 70,000 years total) and to marine phreatic conditions during high stands of the sea (< 35,000 years total). Despite prolonged exposure to vadose conditions, diagenetic modification is slight. If the recrystallized lime muds contain greater than 25 percent cement (micrite), then mass-balance calculations suggest large amounts of calcium carbonate have been transported to that part of the sedimentary section exposed to freshwater phreatic diagenetic processes. The source of the additional carbonate may be updip in the aquifer, from mixing of underlying marine pore fluids with the freshwater pore fluids of the coastal water lens, from the overlying shallow vadose zone, or from marine water soon after deposition. Processes which affect early diagenetic modification of carbonate sediments on the south coast of Barbados operate more rapidly and more effectively in the freshwater phreatic environment than in vadose or marine phreatic environments.

Controls on the oxygen and hydrogen isotopic composition of the waters of Florida Bay, U.S.A.

The oxygen and hydrogen isotopic composition have been measured of waters in Florida Bay and fluids squeezed from sediments which make up Holocene islands in the bay. Although, these waters ranged in salinity from 27 to 120 g kg−1, most were found to have very similar hydrogen and oxygen isotopic compositions (δ18O = + 2 to + 4‰, δD= + 5 to + 25‰). In order to explain these observations we have applied the Craig-Gordon and Gonflantini evaporation models which account for oxygen and hydrogen isotopic fractionation during the desiccation of saline water bodies. These models provide excellent agreement for the evaporation of water into an environment with a relative humidity of 77%, a temperature of 25°C and atmospheric water vapor possessing δ18O- and δD-values of −11 and −75‰, respectively. The salinity of fluids from one core on Cluett Key (26.9 g kg−1) was well below that of the surrounding bay (> 40 g kg−1 and the water from this locality was depleted in both D and 18O. These δ18O- and δD-values, plotted together with other data from this study, fall on a line possessing a slope of 4.90 ( ± 0.41), similar to what would be expected from the evaporation models. This line intersects the meteoric water line at δ18O and δD-values which are within error, similar to average δ18O- and δD-values measured for rainfall in the Miami area.

Phreatic vs. vadose diagenesis; stratigraphy and mineralogy of a cored borehole on Barbados, W. I

ABSTRACT Meteoric water diagenesis may take place in the vadose-subaerial environment or in the fresh water phreatic environment. This paper presents data to suggest meteoric water diagenesis may occur more rapidly in the phreatic diagenetic environment. Continuous drill core was recovered from a borehole in the radiometrically dated 105,000 years Before Present (B.P.) reef tract on the south coast of Barbados, W. I. Diagenetic environments have passed through the sediment of this borehole in response to Pleistocene glacio-eustatic fluctuations of sea level and gradual tectonic uplift of the island. Carbonate sediments from the upper 1.5 m of the borehole have been in the vadose diagenetic zone since initial emergence from the depositional environment. These carbonate sediments are for the most part mineralogically unstable; they have not been affected by extensive dissolution. Fresh water phreatic lenses associated with high stands of sea level have occupied the pore systems of the sediments between 1.5 m and 15.3 m depth at least twice and possibly three times. Those zones occupied at least once by a fresh phreatic lens show complete mineral stabilization of the inter-coral matrix to low-Mg calcite, and extensive cementation and dissolution. Sediment from the lowest part of the core still retains most of its depositional mineralogy and shows only minor dissolution or cementation. This portion of the borehole has alternately been subjected to the vadose zone (during glacial low stands of the sea) and the marine phreatic zone (during interglacial high stands). Although interpretations of sea level fluctuations indicates the sediment from this borehole spent more time in the vadose diagenetic environment than all other environments combined, it is significant that only those portions of the borehole section once occupied by fresh phreatic pore fluids show extensive diagenetic modification. Indeed, the lower portion of the borehole section, despite prolonged exposure to the vadose diagenetic environment, shows only minor diagenetic modification. Thus on the south coast of Barbados, early carbonate diagenesis takes place more rapidly in the fresh water phreatic zone than in the other diagenetic environments combined.

SEM observations on the replacement of Bahaman aragonitic mud by calcite

Conversion of Holocene aragonitic mud to calcite microspar occurs in fresh pore waters beneath hammocks on the tidal flats of west Andros Island, Bahamas, to a subsurface depth of 2.7 m. The conversion process involves both dissolution of aragonite and precipitation of calcite as a cement in pre-existing pores and displacive calcite crystallization. Neomorphism of aragonite to calcite with preservation of grain shape has not been observed. Resulting diagenetic products are lithified nodules, composed of well-sorted interlocking rhombs of microspar and of stiff, unlithified calcitic aragonite mud.

On the diagenesis of lime mud; scanning electron microscopic observations of subsurface material from Barbados, W.I.

ABSTRACT Diagenesis of lime mud involves mineralogic stabilization, addition of large amounts of porosity-reducing cement and/or compaction, and grain shape changes, processes that may occur rapidly when fresh water invades the sediment pore systems. Scanning electron microscopy of lithified and semilithified Pleistocene lime mudstone from a subsurface location on Barbados (Borehole #17) illustrate some of these early diagenetic processes. Mineralogically stable muds that have resided in fresh water phreatic lenses are well cemented with porosity less than 15%. Mud particles are 0.5 to 2 µm in diameter Three to 8 µm grains of cement occur in aggregates lining irregular coarse silt- and sand-sized cavities (molds?). In the modern zone of mixing of fresh and marine ground water. mineralogically metastable muds contain 20-30% calcite crystals, 3 to 5 µm in size, partially filling micropores and cementing 0.25 to 2 µm mud particles; porosity is 20-40%. These data indicate that early cementation of lime mud can occur in fresh and brackish phreatic environments. Mineralogic stabilization of the muds involves dissolution of 0.25 to 2 µm sized metastable grains and precipitation of calcite crystals up to 8 µm in size. Interpretation of the scanning electron micrographs indicates that aggrading neomorphism has not been an important process in the early evolution of the mud fabric.

Lime-mud layers in high-energy tidal channels: a record of hurricane deposition

During or immediately following the transit of Hurricane Andrew (August 23-24, 1992) across the northern part of the Great Bahama Bank, thin laminated beds of carbonate mud were deposited in high-energy subtidal channels (4 m depth) through the ooid shoals of south Cat Cay and Joulters Cays. During our reconnaissance seven weeks later, we observed lime-mud beds exposed in the troughs of submarine oolite dunes and ripples. The mud layers were underlain and locally covered by ooid sand. The mud beds were lenticular and up to 5 cm thick. Their bases cast the underlying rippled surface. The layers were composed of soft silt- and sand-sized pellets and peloids and in some areas contained freshly preserved Thalassia blades and other organic debris along planes of lamination. The beds had a gelatinous consistency and locally had been penetrated by burrowers and plants. Layers of lime mud had also settled on bioturbated, plant-stabilized flats and in lagoonal settings but were quickly reworked and made unrecognizable by the burrowing of organisms. Thicker, more cohesive (and therefore older) mud beds and angular mud fragments associated with ooids from Joulters Cays have similar characteristics but lack fresh plant fragments. We infer that these older beds were similarly deposited and thus record the passage of previous hurricanes or tropical storms. Storm layers are preserved within channel sediments because migrating ooids prevent attack by the burrowing activity off organisms.

A Mesozoic Carbonate Hot-Spring Deposit in the Hartford Basin of Connecticut

ABSTRACT Limestone at Coes Quarry in North Branford, Connecticut, is characterized by a variety of cross-cutting, discontinuous textural facies. The environmental significance of most textural features is ambiguous, but cellular tufa and abundant spherulite strongly suggest a boiling hot spring setting. The cellular tufa consists of a thin, polygonal framework of micritic calcite which surrounds and interconnects irregular elongate voids. The void space is filled with spherulites and locally derived carbonate detritus cemented by sparry calcite and dolomite. Spherulites nucleated and grew in suspension to diameters of 0.1 mm or more before settling in the cellular tufa and other nearby depositional settings. Spherulites may form distinct geopetal fabrics in the pores of the cellular tufa, dem nstrating that some indistinct layering in tufa had high primary dips. Other important depositional facies are algal/bacterial tufa, micritic and banded travertine, and siliciclastic, limy sandstone. All units are abruptly discontinuous laterally and are also found as intraclasts mixed in with other facies. The spherulites indicate rapid precipitation of carbonate from highly supersaturated bicarbonate waters formed by flash-boiling of CO2-rich water. The cellular textures are similar to microterrace deposits and encrusted organic grains and mats in modern hot springs. Saddle dolomite, which is an important early cement, suggests temperatures of at least 60°C. We infer that the limestone was deposited from a hot spring which periodically flash-boiled in violent, geyserlike eruptions. The Talcott Basalt, which immediately underlies the limestone, is, in places, totally replaced by ferroan carbonates. We infer that the same circulating groundwater, heated by deeper, intrusive equivalents of the Talcott Basalt, locally altered the basalt and reached the surface at hot springs like the one from which the limestone at Coes Quarry precipitated.