Brian White

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.

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.

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.

Development and Decline of a Silurian Stromatolite Reef Complex, Glacier Bay National Park, Alaska

Abstract In Glacier Bay, Alaska, Silurian limestones record the development and demise of a stromatolite reef complex in the Alexander terrane. These microbial deposits are of regional and paleontological significance because they contain paleogeographically distinctive biotas and yield important insights into Phanerozoic stromatolites that inhabited normal-marine subtidal environments. Willoughby limestones exposed on Drake Island reveal that stromatolite growth at the platform margin influenced platform dynamics with the protection of peritidal and lagoonal habitats behind a reef-fringed rim, which experienced early lithification by the precipitation of synsedimentary marine cements. Relatively low-energy subtidal conditions in a restricted, shallow-marine lagoon are implied by the peloidal and mollusk-rich wackestones, packstones, and grainstones. At the platform margin, stromatolite boundstones and cementstones capped a reef-like mound by forming a thick microbial-cement crust on a core of outer lagoonal sediments. Well laminated, fenestral mudstones and peloidal wackestones-grainstones have an abrupt lateral contact with the microbial boundstones, indicating their formation in peritidal flats adjacent to the stromatolite reefs. Foreslope breccias consist of clasts derived from shallow subtidal environments on the shelf and at its rim. Cyclic repetition of these deposits indicate that sea level fluctuations and tectonic instability in the Alexander terrane contributed to periodic shedding of shallow-marine detritus to the upper slope, followed by the rebuilding of stromatolite reefs at the platform margin. Culminating stages in the Klakas orogeny induced large-scale, catastrophic collapse of the carbonate platform. Silurian stromatolites characterized by similar habitats and microbial-sponge biotas in southwestern Alaska and the Ural Mountains, Russia, together with paleomagnetic data, detrital zircon evidence, and the timing of late-stage Caledonide events, firmly establish genetic and temporal links of the Alexander terrane with other areas along the Uralian Seaway.

Paleoecology of Sponge-?Hydroid Associations in Silurian Microbial Reefs

Abstract Microbial boundstones from Alaska and Russia yield new insights into the paleoecology of Silurian biotas that inhabited stromatolite reefs. These high-energy reefs were built along the Uralian Seaway in the Late Silurian by a diverse suite of microorganisms in association with accessory metazoans, predominantly sphinctozoan sponges. Within the stromatolite framework, three species of small, solitary, sphinctozoans (aphrosalpingids) encrusted a variety of hard substrates, mostly skeletal remains but also microbial laminae and cavity surfaces. Fossils encrusted by the sponges include the problematic hydroid Fistulella, possible stromatoporoids (recrystallized), crinoids, the possible cyanobacterium Ludlovia, corals, and unidentifiable shelly debris. In addition to the ubiquitous microbial laminae, the sponges, Fistulella, and ?stromatoporoids were less commonly encrusted by Ludlovia, Renalcis, or crinoids. Well-developed attachment surfaces, including enlarged holdfasts, allowed the sponges to achieve stability on the seafloor after larvae settled randomly on available hard surfaces. A greater incidence of sponge encrustations on Fistulella than on other organisms indicates that some of the sponges may have enjoyed a commensalistic relationship while attached as juveniles to a living substrate. The sponges’ orientation on Fistulella in the sediment suggests that the relationship between the two taxa may have become parasitic, whereby the weight of the sponges caused Fistulella to collapse into the muddy substrate. Recognition of the intimate growth relationships shared by Silurian sphinctozoans, Fistulella, and other organisms expands the fossil record of encrusting sponges, identifies a novel sponge-?hydroid association, and reveals organismal responses to competition for space in mid-Paleozoic microbial reefs.

Stromatolites and associated facies in shallowing-upward cycles from the Middle Proterozoic Altyn Formation of Glacier National Park, Montana

Abstract The approximately 1450 Ma old Altyn Formation is the oldest stratigraphic unit exposed in Glacier National Park, Montana. The lower part of the formation, informally called member A, contains at least 17 shallowing-upward cycles, with an average cycle thickness of 4.2 m. Each cycle rests on an erosional surface and has subtidal quartz-microcline-dolarenites in the basal part. The subtidal rocks pass upward into stratiform stromatolites interbedded with dolarenites similar to those in the basal layers. The stromatolites formed on tidal flats during calm periods and the dolarenites were transported from subtidal areas to the tidal flats by storm-driven waves and currents. Domal and columnar stromatolites grew in relatively higher energy conditions, particularly in tidal channels in the lower tidal flats. The upper parts of some cycles have sparse dolarenites and are dominated by stratiform stromatolites. Desiccation features and complete dolomitization of subtidal and intertidal carbonate sediments occur in member A. Many cycles within this member contain evidence of the diagenetic formation of gypsum and anhydrite in sediments deposited in subtidal and intertidal zones. The carbonate sediments in each cycle accumulated up to sea level causing progradation of tidal flats into a shallow subtidal area. This led to the development of a supratidal zone analogous to a modern sabkha. Member A is overlain by quartz—microcline-dolarenites of informal member B, which were deposited in subtidal environments. Informal member C overlies member B and contains shallowing-upward cycles similar to those of member A, but with an average cycle thickness of 2.5 m. Member C cycles also differ in containing mud-cracked shales and terrigenous arenites. These terrigenous sediments were derived by mechanical weathering in the source area. They were transported into the supratidal and upper intertidal zones during flash flooding. The change from member A to member B represents an overall transgressive trend. This was followed by an overall regression giving the change from member B to member C. The repeated shallowing-upward cycles are due to smaller transgression—progradation events that were superimposed on these larger trends. The Altyn Formation was deposited in an epicontinental basin. Each shallowing-upward cycle was produced by sediment aggradation and progradation, following rapid subsidence resulting from vertical tectonic activity. The paleoclimate was arid, with occasional storms flooding the tidal flats with subtidal sediments and the upper tidal flats with terrestrial sediments. Such a paleoclimate is consistent with the subtropical to tropical paleolatitudes indicated by paleomagnetic data for the Altyn Formation depositional area.

Taphonomy and paleobiological implications of Middle Devonian (Eifelian) nautiloid concentrates, Alaska

Significant accumulations of microscopic nautiloids in Devonian carbonate grainstones are reported for the first time from the fossil record. Small-sized nautiloids belonging to three or four taxa are the predominant skeletal components in shell concentrates that are nine m thick. The nautiloids have an average length <6 mm and an average diameter <2 mm, are densely packed, and moderately well sorted with rare evidence of grading, imbrication, or alignment. Association with abundant massive stromatoporoids and colonial corals indicates that the nautiloid communities inhabited a shallow-marine reefal environment. The extraordinary abundance of small-sized nautiloids, their high density and concentration, as well as the thickness and repeated occurrence of the nautiloid grainstones in the reefal sequence suggest that a combination of biological (group spawning and mass mortality) processes and physical (storm-related) phenomena were associated with the production, concentration, and preservation of an unusually high abundance of small adult and juvenile nautiloids. The inferred life history strategies of these Devonian nautiloids suggest significant differences from the modern Nautilus but remarkable similarities with belemnites, ammonites, and modern coleoids.