H. Allen Curran

Prominence of ichnologically influenced macroporosity in the karst Biscayne aquifer: Stratiform "super-K" zones

A combination of cyclostratigraphic, ichnologic, and borehole geophysical analyses of continuous core holes; tracer-test analyses; and lattice Boltzmann fl ow simulations was used to quantify biogenic macroporosity and permeability of the Biscayne aquifer, southeastern Florida. Biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating from dissolution of fossil hard parts, principally mollusk shells. Ophiomorpha-dominated ichnofabric provides the greatest contribution to hydrologic characteristics in the Biscayne aquifer in a 345 km 2 study area. Stratiform tabular-shaped units of thalassinidean-associated macroporosity are commonly confi ned to the lower part of upward-shallowing highfrequency cycles, throughout aggradational cycles, and, in one case, they stack vertically within the lower part of a high-frequency cycle set. Broad continuity of many of the macroporous units concentrates groundwater fl ow in extremely permeable passageways, thus making the aquifer vulnerable to long-distance transport of contaminants. Ichnogenic macroporosity represents an alternative pathway for concentrated groundwater fl ow that differs considerably from standard karst fl ow-system paradigms, which describe groundwater movement through fractures and cavernous dissolution features. Permeabilities were calculated using lattice Boltzmann methods (LBMs) applied to computer renderings assembled from X-ray computed tomography scans of various biogenic macroporous limestone samples. The highest simulated LBM permeabilities were about fi ve orders of magnitude greater than standard laboratory measurements using air-permeability methods, which are limited in their application to extremely permeable macroporous rock samples. Based on their close conformance to analytical solutions for pipe fl ow, LBMs offer a new means of obtaining accurate permeability values for such materials. We suggest that the stratiform ichnogenic groundwater fl ow zones have permeabilities even more extreme (~2‐5 orders of magnitude higher) than the Jurassic “super-K” zones of the giant Ghawar oil fi eld. The fl ow zones of the Pleistocene Biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of origin and evolution in other carbonate aquifers, as well as petroleum reservoirs.

Complex decapod burrows and ecological relationships in modern and Pleistocene intertidal carbonate environments, San Salvador Island, Bahamas

Abstract Burrowing by thalassinidean shrimp in modern tropical, shallow-subtidal to intertidal carbonate environments is widespread and a powerful agent of bioturbation, particularly within sandy substrates. Deep and intense burrowing by callianassids commonly occurs along the intertidal margins of hypersaline lagoons throughout the Bahamas, such as Pigeon Creek on San Salvador Island, where the sediments of extensive sand flats are thoroughly bioturbated by Glypturus acanthochirus . In addition to dominating the deep-tier infauna, G. acanthochirus is a true ecosystem engineer, and its prodigious burrowing activity results in a highly mounded topography. On sand flats, individual sediment cones commonly coalesce with time to form large composite mounds with surfaces stabilized by the development of microbial mats. This unique, biogenetically produced topography sets the stage for colonization by shallow-tier burrowers, particularly the upogebiid shrimp Upogebia vasquezi and several species of fiddler crabs. Burrow systems of U. vasquezi are distinctive and complex. They typically consist of a pair of U-shaped burrows in close proximity or criss-crossing, with knobs or short tunnels at their bases. The entire U-burrow pair is lined by a thick, externally pelleted wall. Inside burrow diameters range from 0.2 to 1 cm, and burrow depths are 10–15 cm, in contrast to the much deeper callianassid burrows. These complex upogebiid burrows also occur as trace fossils in late Pleistocene lagoon-margin facies of the Grotto Beach Formation on San Salvador. Initially reported as fragmentary burrow fills, whole burrow systems virtually identical to the modern upogebiid burrows recently have been found at several localities. These fossil burrows are most abundant in beds lying immediately below a terra rossa paleosol that marks the late Pleistocene–Holocene stratigraphic boundary, and the beds likely were deposited with sea-level stillstand and regression associated with the onset of Wisconsinan glaciation. With their high potential for preservation, these complex upogebiid trace fossils may prove useful as both paleoenvironmental and sea-level position indicators in the study of Quaternary carbonate sequences throughout the Bahamas and other geologically similar regions.

Trace fossils of shallow subtidal to dunal ichnofacies in Bahamian Quaternary carbonates

Pleistocene and Holocene carbonate grainstones or calcarenites capping the islands of the Bahamas commonly contain distinctive animal and/or plant trace fossils. Three ichnocoenoses within the Skolithos and Psilonichnus ichnofacies are recognized in the transition from sediments deposited in the shallow shelf environment, commonly associated with coral reefs, to sediments of the coastal dune environment. Analog relationships between the trace fossils and modern tracemakers can be established in many cases, and this correspondence strengthens the interpretive model. Ophiomorpha and Skolithos linearis characterize beds deposited in shallow shelf settings. Psilonichnus upsilon, the fossil burrow of the ghost crab Ocypode quadrata, marks beds deposited in the upper foreshorebackshore environment and has particular utility as an indicator of sea-level position. A diverse ichnocoenosis consisting of Skolithos linearis, a large cluster burrow, small, irregular burrows, and plant trace fossils formed along bedding planes characterizes beds of the dunal environment. The ichnologic model developed herein for recognition of depositional zones in the transition from shallow subtidal to dune environments in the tropical, carbonate coastal settings of the Bahamas should be applicable to other geologically similar settings around the world.

Black Shell turbidite, Hatteras Abyssal Plain, western Atlantic Ocean

An upper Pleistocene turbidite with a volume of at least 1011 m3 (100 km3) has been traced in 35 piston cores over a 44,000 km2 area of the Hatteras Abyssal Plain. Entering the plain from the Hatteras Canyon System, the flow traveled uninterrupted for at least 500 km in a southerly direction and resulted in a tongue-shaped turbidite with a width between 100 and 140 km and a thickness of as much as 400 cm. This turbidite appears to be one of the largest, if not the largest, single turbidite yet to be correlated between deep-sea cores and to be mapped on an abyssal-plain floor. Correlation of the unit is based on grain size, mineralogy, relative thickness, and similarity of sequences in the cores. Because correlation between cores is based largely on sand-layer characteristics, the turbidite cannot be traced beyond the last occurrence of a distinct sand layer in the distal (southerly) direction. The turbidite is characterized by its high percentage (2% to 50%) of blackened mollusk shell fragments, which led to the informal name Black Shell turbidite, and by a coarser grain size than other turbidites in the same cores. The maximum thickness of the turbidites sand part is in the center of the abyssal-plain basin, whereas maximum thicknesses of lutite and of the total turbidite are displaced east of the center line of the depositional basin. Depending on lateral position in the flow, the sands texturally comprise a wide range of graywackes, and mineralogically they constitute a suite ranging from lithic arkoses to quartzarenites. Sand petrology indicates that the fluvially derived terrigenous fraction came from United States mid-Atlantic coast rivers, whereas molluscan and foraminiferal bioclastic components indicate an initial shallow-shelf origin from the vicinity of Cape Hatteras, North Carolina. It appears that the turbidity current began as a massive shelf-edge slump. Sand came from the shelf edge; mud was picked up on the upper continental slope. The flow apparently evolved from a slump into a high-concentration flow as it moved down-slope. Characteristics in the axial center zone, such as poor sand sorting, high mud content, Bouma AE sequences, and discontinuous vertical grading, suggest deposition from a high-concentration flow. The nature of the characteristics changed (better sorting, more continuous vertical grading, more complete Bouma sequences, and lower mud content) as the flow spread, reflecting deposition from a low-concentration turbidity current.

Biogenic Sedimentary Structures Produced by Crabs in Lagoon Margin and Salt Marsh Environments Near Beaufort, North Carolina

ABSTRACT Intertidal burrowing crabs produce distinctive biogenic sedimentary structures in protected lagoon margin and salt marsh environments in the vicinity of Beaufort, North Carolina. Distribution of crabs is determined primarily by substrate characteristics, salinity, and vegetation cover in the intertidal zone. Protected sand beaches and flats are heavily burrowed by the sand fiddler, Uca pugilator, which produces gently curved J-shaped and sharply curved L-shaped burrows 1 to 2 cm in diameter. Larger Y- and U-shaped burrows are formed by the ghost crab, Ocypode quadrata, in the backshore and foredune ridge zones above the high tide line. Salt marshes influenced by normal to near normal marine salinities are burrowed by U. pugilator, U. pugnax, and Sesarma reticulatum. U. pugnax and S. reticulatum burrow in muddy substrates, particularly along tidal creek banks, whereas U. pugilator is restricted to sandy substrate areas. U. pugnax burrows are 1 to 2 cm in diameter and have complex twisting and turning forms. The larger communal burrows of S. reticulatum consist of a complex of interconnected shafts with several oval-shaped surface openings. Panopeus herbstii is often found inhabiting these burrows. In salt marshes where the influence of fresh water is great, U. minax and U. pugnax are the dominant crabs. U. minax forms burrows 2 to 5 cm in diameter with ho ded entrances and long vertical shafts. Intertidal crabs are important agents of bioturbation in the environments that they inhabit. Their burrows have excellent potential for preservation in the sedimentary record. Recognition of the record of biogenic sedimentary structures produced by intertidal crabs would be of significant value in identifying paleoenvironments, fixing ancient lagoon margin and estuarine shorelines, and determining paleosalinities.

Bahamian coral reefs yield evidence of a brief sea-level lowstand during the last interglacial

The growth of large, bank-barrier coral reefs on the Bahamian islands of Great Inagua and San Salvador during the last interglacial was interrupted by at least one major cycle of sea regression and transgression. The fall of sea level resulted in the development of a wave-cut platform that abraded early Sangamon corals in parts of the Devils Point reef on Great Inagua, and produced erosional breaks in the reefal sequences elsewhere in the Devils Point reef and in the Cockburn Town reef on San Salvador. Minor red caliche and plant trace fossils formed on earlier interglacial reefal rocks during the low stand. The erosional surfaces subsequently were bored by sponges and bivalves, encrusted by serpulids, and recolonized by corals of younger interglacial age during the ensuing sea-level rise. These later reefal deposits form the base of a shallowing-upward sequence that developed during the rapid fall of sea level that marked the onset of Wisconsinan glacial conditions. Petrographic studies reveal a diagenetic sequence that supports this sea-level history. Preservation of pristine coralline aragonite, coupled with advances in U/Th age dating, allow these events in the history of the reefs to be placed in a precise chronology. We use these data to show that there was a time window of 1,500 years or less during which the regression/transgression cycle occurred and that rates of sea-level change must have been very rapid. We compare our results with the GRIP ice-core data, and show that the history of the Bahamian coral reefs indicates an episode of climate variability during the last interglacial greater than any reported in what is widely believed to be the more stable climate of the Holocene interglacial.

Ichnofacies, Ichnocoenoses, and Ichnofabrics of Quaternary Shallow-Marine to Dunal Tropical Carbonates: A Model and Implications

SUMMARY: A model of five ichnocoenoses within the Skolithos and Psilonichnus ichnofacies characterizes the modern, Holocene, and Pleistocene coastal-carbonate depositional environments and limestones of the Bahamas, as well as the Miami Limestone of south Florida. The subtidal to intertidal ichnocoenoses of the Skolithos ichnofacies are dominated by trace-making activities and trace fossils of callianassid shrimp, which can create distinctive and maximum ichnofabrics. Fossil Upogebia vasquezi burrows found in intertidal calcarenites and burrows of the trace fossil Psilonichnus upsilon, most common in beach backshore beds, have excellent potential as stratigraphic markers and can be used as indicators of past sea-level positions. The dunal ichnocoenosis exhibits a high ichno-diversity owing to the presence of arthropod-generated trace fossils and rhizomorphs, resulting from the activities of plants; trace fossils created by insects can be large and complex and can impart distinctive ichnofabrics to eolianites. Ichnologic studies of modern tropical carbonate environments and their rock-record equivalents have great potential for future development, and information from carbonates should be fully integrated with that of siliciclastics, with carbonates not viewed as a separate ichnologic subdiscipline.

Mesoscale physical sedimentary structures and trace fossils in Holocene carbonate eolianites from San Salvador Island, Bahamas

Abstract Carbonate eolianites, less than 10,000 years old, are well exposed in sea cliffs and on rocky shore platforms along the northeast coast of San Salvador Island. These deposits formed when easterly trade winds blew carbonate sands landward from the beach zone as sea level rose over a previously exposed shelf during the Holocene transgression. Small, lobate, parabolic-like dunes coalesced laterally to form an elongate, transverse dune ridge oriented perpendicular to the prevailing wind direction. Detailed observations of small-scale sedimentary structures and laminations permit the distinction of sands deposited as climbing wind ripples, lee-side grainfalls, and lee-side sandflows. Micrite crusts and associated plant trace fossils characteristic of the dunal environment are common in the Rice Bay Formation, and these can be compared directly with identical features and plants found in modern carbonate dunes on San Salvador. Some eolian laminations dip into the present-day subtidal zone, confirming a post-depositional rise in sea level along this tectonically stable coast. The rocks are lithified sufficiently by freshwater, vadose, low Mg-calcite cement to form wave-resistant sea cliffs. A distinctive feature of these terrestrial carbonate rocks is the occurrence of a variety of animal trace fossils. Skolithos linearis burrows, up to 30 cm in length and 0.5 cm in diameter, are quite common. Comparison with traces found in modern carbonate dunes suggests that these trace fossils were made by burrowing insects or spiders. The most abundant and widespread trace fossil consists of closely spaced, irregular, small burrows up to 20 cm long in the horizontal plane and with uniform diameters of 0.3–0.4 cm. These burrows also extended downward as much as 3 cm into the sediments, and created a mottled texture in places. This trace fossil has been found only in lee-side sandflow and grainfall deposits. Such burrows probably were produced by insects or insect larvae, which favored the protected lee-side environment. The most unusual trace fossil in these Holocene eolianites is composed of a cluster of vertically oriented burrows. Each cluster consists of up to several hundred shafts, each with a diameter of 1–2 cm, that diverge upward from an approximately common point of origin. These structures can be 1.4 m or more high and greater than 1 m in transverse section across the circular shape produced by the upwardly radiating burrow cluster. These trace fossils probably represent the escape pathways of the hatchlings of an infaunal insect or similar invertebrate.

Shallow-Marine Carbonates

Abstract The ichnology of carbonate sedimentary systems is less well studied compared to that of siliciclastic systems. Shallow-marine carbonates differ from their siliciclastic counterparts by a number of criteria, such as the composition and distribution of trace-fossil associations, broad and very shallow facies belts with intense bioturbation and subaerial exporsure, and a strong impact of diagenetic processes on carbonate sediments. Early cementation typically leads to the occurrence of firmgrounds and hardgrounds with intense bioerosion and preferential trace-fossil preservation. The Psilonichnus, Skolithos, and Cruziana ichnofacies are important for the reconstruction of paleoenvironments on carbonate shelves (or ramps), whereas the substrate-controlled Glossifungites and Trypanites ichnofacies can overprint the former ones during periods with stagnant or low net-sediment accumulation. The value of ichnological analysis in shallow-marine carbonates is demonstrated on Middle Ordovician cool-water carbonates in Russia, Middle Triassic epicontinental carbonates in Germany, the Quaternary rimmed carbonate platform of the Bahamas, and the Quaternary homoclinal carbonate ramp of the southern Arabian Gulf.

Problems in Interpreting Unusually Large Burrows

Although marine burrows of unusually large dimensions have long been known in certain areas, they are probably much more widespread in the rock record than is generally recognized. Such burrows constitute a heterogeneous group, having little in common other than “exceptional” size. Yet their size alone unites them in difficulty of interpretation: e.g., densely spaced?dwelling burrows or combined dwelling-escape burrows as much as 12 cm in diameter and 5 m long; vertical dwelling burrows only 0.5 cm in diameter but up to 9 m long; possible escape structures as much as 24 cm in diameter and 3 m long, subsequently penetrated in some cases by secondary burrow-like structures.