Geological Society of America,
North American Paleontological Convention,
and
American Geophysical Union Abstracts
from the
Sedimentology, Stratigraphy and Paleontology Program
at
The College of Wooster
1999-2001

CARBONATE HARDGROUNDS AND THEIR FAUNAS IN A COARSE SILICICLASTIC ENVIRONMENT: THE QAHLAH FORMATION (UPPER CRETACEOUS) OF THE OMAN MOUNTAINS

WILSON, Mark A., Dept of Geology, The College of Wooster, Wooster, OH 44691 USA, mwilson@acs.wooster.edu; TAYLOR, Paul D., Dept of Palaeontology, The Natural History Museum, London, SW7 5BD, UK, pdt@nhm.ac.uk.

There is normally an antithetical relationship between carbonate hardgrounds and coarse siliciclastic sediment. Skeletal encrusting organisms are usually rare in such environments because of high levels of abrasion. An exception to this is the Maastrichtian Qahlah Formation of the Oman Mountains, a sequence of coarse sandstones and gravels deposited on a shallow marine shelf above wavebase. The siliciclastics contain intercalated carbonate hardgrounds and many other hard substrates which were variously encrusted and bored. These include carbonate and silicic clasts, calcareous shells (especially of rudistid and other mollusks), and wood. The diverse hard substrate fauna is dominated by the oyster Acutostrea, which is often densely aggregated on upward-facing surfaces. Other encrusters on exposed surfaces include agglutinated foraminifera and several species of colonial corals. There are more than ten bryozoan species, all of which were cryptic and relatively rare. Two species of serpulids were also cryptic. Borings in the carbonate clasts and shells include those of bivalves (Gastrochaenolites), clionid sponges (Entobia) and acrothoracican barnacles (Rogerella). The woodgrounds are thoroughly bored by teredinid bivalves, which produced Teredolites. The Qahlah hard substratum faunas can be categorized as robust skeletal encrusters (oysters and corals), cryptic encrusters (bryozoans and serpulids found on undersides, within vacated borings or prelithification burrows) and endolithic taxa (bivalves, sponges and barnacles). Delicate agglutinated foraminifera are found anomalously on exposed surfaces, sometimes covering truncated borings. These foramininfera may have recruited quickly enough to maintain a presence on the clast exteriors despite the abrasion, or they may have lived interstitially on stable clasts below the sediment-water interface.

BIOIMMURATION AS A KEY TO PALEOECOLOGY ON SHELL SUBSTRATES AND EARLY ARAGONITE DISSOLUTION IN A CALCITE SEA (UPPER ORDOVICIAN, CINCINNATI REGION, USA)

LAZZURI, Jessica E., Department of Geology, Beloit College, Beloit, WI 53511; FISCHER, Woodward W., Geology Department, Colorado College, Colorado Springs, CO 80903; WILSON, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691; TANG, Carol M., Department of Geology, Arizona State University, Tempe, AZ 85287

Bioimmuration is the process by which sessile animals or plants are molded within the mineralized skeletons of organisms which overgrew them. The limestones and claystones of the Upper Ordovician (Cincinnatian) sequence in Indiana, Kentucky and Ohio contain abundant examples of aragonitic shells and soft-bodied organisms bioimmured by encrusting stenolaemate bryozoans. The fast-growing bryozoans spread along the interiors and exteriors of aragonitic mollusk shells, molding the substrate and other encrusters underneath their calcitic skeletons. Boring organisms of several types cut through the encrusting bryozoans and into the aragonitic shells beneath. The aragonite quickly dissolved on the seafloor. The exposed colony base records the bioimmurations, and was often itself encrusted by additional organisms. Some bryozoans overlapped the edges of the aragonitic shell and continued to encrust their own basal layers, showing that aragonite dissolution took place within the lifespan of a single bryozoan colony. The borings were often cast by early-cemented infilling sediment, preserving excavation details not usually visible. These casts were exposed after aragonite dissolution and sometimes encrusted. Many borings penetrated living bryozoans, producing a variety of skeletal responses by the host. The borings often started perpendicular to the colony surface, but when they reached the aragonitic shell beneath they moved laterally along the aragonite-calcite interface.
This unique view of the bases of the encrusting bryozoans shows the order and patterns of shell encrustation. Runner-type bryozoans were usually the first on the substrates, followed by thin sheet-like encrusting cystoporates and trepostomes, and then thicker ramose trepostomes. Internal molds of aragonitic bivalves, particularly Ambonychia, show that the runner-type cyclostome bryozoan Cuffeyella was almost exclusively cryptic, whereas others were always on exposed surfaces.

PALEONTOLOGICAL AND ISOTOPIC FEATURES OF AN UNUSUAL SEA LEVEL EVENT RECORDED IN FOSSIL CORAL REEFS OF THE BAHAMAS (SUBSTAGE 5E, EEMIAN, SANGAMONIAN, PLEISTOCENE)

CORNETT, Allison M., cornetam@acs.wooster.edu, and WILSON, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691, mwilson@acs.wooster.edu.

The beautifully preserved fossil reefs of the Bahama Islands are ideal for the study of eustatic sea-level change, due to their tectonic stability and low, regular subsidence rate. Recent description of an erosion surface propagating through Bahamian reefs formed during the Eemian interglacial has led to a reevaluation of sea-level stability at that time. This surface has been described on the islands of San Salvador and Great Inagua; such a broad occurrence over an isostatically stable area is indicative of an eustatic regressive and transgressive sequence. Supporting paleontological evidence for this sea-level event includes truncated corals, borings, encrusters, rhizomorphs, and an extensive paleosol. U-Th dating of corals directly above and below the erosion surface has constrained this event to within 1,500 years, approximately 125 ka. As eustatic sea-level events are most often the result of changes in global ice volume, this event calls into question the stability of the Eemian interglacial climate. A proxy background signal for the interglacial climate has been developed by examining stable isotopes of corals from the two reef-building episodes. Samples were collected from the Eemian reefs on Great Inagua and San Salvador, both above and below the disconformity, for analysis of their stable oxygen and carbon isotopes. Microsamples of a massive Diploria strigosa truncated by the erosion surface are also being analyzed for stable isotopes; the signal leading up to its death is a record of climate change during the onset of regression. These analyses will help describe the mechanism for such an unusual sea-level event and related climate fluctuations. This work may be useful in furthering knowledge of quickly changing climate systems and their global effects.

PALEOENVIRONMENTS OF THE SIMSIMA FORMATION (MAASTRICHTIAN, LATE CRETACEOUS), OMAN MOUNTAINS, ARABIAN PENINSULA

HOOKER, Megan F., hookermf@acs.wooster.edu and WILSON, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691, USA, mwilson@acs.wooster.edu; TAYLOR, Paul D., Department of Palaeontology, The Natural History Museum, London, SW7 5BD, UK, pdt@nhm.ac.uk.

The Late Cretaceous (Maastrichtian) Simsima Formation is a fossiliferous carbonate unit deposited in the ancient tropical Tethys during a general marine transgression. The Simsima is located on the southeastern Arabian Peninsula in present-day Oman and the United Arab Emirates. It has been studied primarily for the purpose of oil exploration, and as a result much of the fluctuating paleoenvironmental details of the formation have not yet been fully explored. The Simsima Formation was deposited mostly below the wave base with numerous rudist clam build-ups. Sedimentary petrography shows a variety of alternating biomicrites and biosparites, with facies generally deepening upwards. Important fossil grains include rudist fragments, discocyclinid and miliolid foraminiferans, echinoderm fragments, gastropods, and dasycladacean and red algae. The Simsima Formation is contemporaneously faulted in some areas, showing that the tectonic setting of the region was unstable during and after Late Cretaceous deposition. Petrological analysis of the formation confirms this by the presence of many siliciclastic-rich limestones containing biotite, muscovite, magnetite, hematite, spinel and other minerals derived from the Semail Ophiolite. The siliciclastic content of these limestones decreases upwards until the carbonates are nearly pure in composition. Several diagenetic processes are evident in the Simsima, particularly neomorphism, iron oxide replacement, silicification of carbonate grains, and hydrolization of siliciclastic minerals.

PALEOECOLOGY OF BORINGS AND PSEUDOBORINGS IN THE CINCINNATIAN (LATE ORDOVICIAN) OF THE NORTH AMERICAN MIDCONTINENT

WILSON, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691, mwilson@acs.wooster.edu; LAZZURI, Jessica E., Department of Geology, Beloit College, Beloit, WI 53511.

Borings are important components of many Cincinnatian fossil assemblages, but they are often overlooked. Borings represent significant members of paleocommunities which are otherwise unrecorded, and they can provide critical information for paleoecological reconstructions. There are four common boring ichnogenera in the Cincinnatian of the North American midcontinent: Trypanites (a cylindrical excavation), Palaeosabella (similar to Trypanites but distally clavate), Petroxestes (slit-shaped borings by mytilid bivalves), and Ropalonaria (branching ctenostome bryozoan borings). These borings are found in calcitic shells, the remnants of aragonitic shells, carbonate hardgrounds, and carbonate hiatus concretions. They are preserved as original excavations in calcitic substrates, as casts where they were in aragonite, and in semi-relief where they cut the interface between shells and skeletal epibionts such as bryozoans. Trypanites is most common on elevated and exposed surfaces, such as bryozoan mounds on hardgrounds and the convex portions of rugose corals; its density can be used as an estimate of exposure on the seafloor. Palaeosabella is similar to Trypanites in distribution; its clavate portion occasionally held nestlers such as inarticulate brachiopods. Petroxestes is found primarily on carbonate hardgrounds, typically showing the clustered distribution of the mytilid clams which formed them. Some soft-bodied organisms interacted with bryozoans and provoked bryozoan responses which resemble borings but are actually unnamed "pseudoborings". The bryozoan created a raised skeletal rim around the organism and its zoarium. The bryozoans may have been limiting damage with these rims, or they may have been raising a wall to outcompete the presumably filter-feeding organism. In either case, these pseudoborings, like actual borings, preserve a biotic interaction from the Late Ordovician.

The History and Geological Importance of Marine Bioerosion in the Phanerozoic

WILSON, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691.

Bioerosion is the "erosion of substrate by means of biological procedures" (Neumann, 1966). Marine bioerosion includes biotic boring, drilling, rasping, scraping and otherwise reducing hard substrates in the oceans or at their margins. These hard substrates include rock surfaces, early-cemented carbonate hardgrounds, shells, exposed bones, woody plant material, and various artificial substances such as concrete and boat hulls. Marine bioeroders include an astonishing array of organisms, from microbes to mollusks, polychaetes, sponges, crustaceans, sipunculids, chitons, brachiopods, bryozoans, echinoids and fish. Even plant roots can bioerode rocky shorelines. Methods of bioerosion tend to fall into two broad categories: mechanical bioeroders use a series of physical means to excavate a hard substrate, and chemical borers attack primarily carbonate substrates with acids and/or chelating agents. The results of bioerosion are trace fossils when preserved. The most common macroborings (those detectable with a hand lens and larger) are narrow holes produced by worm-like animals, clavate excavations made by bivalves, and small, connected chambers constructed by sponges. Bioerosion trace fossils are very common in the fossil record, and they reflect trends in marine community evolution. Paleozoic and early Mesozoic bioerosion was mostly confined to small, narrow holes, and rates of bioerosion were low. With the beginning of the Mesozoic Marine Revolution, however, bioerosion increased dramatically as numerous animal clades began to exploit infaunal niches. Bivalve and sponge borings became very prominent in carbonate rocks and shells, so much so that significant amounts of substrate were destroyed. Bioeroders from the Cretaceous to today are major players in the sculpting of carbonate shorelines, especially when their work enhances rates of physical erosion. Tropical carbonate shorelines in particular show notching and terracing which is primarily the effect of shallow and deep bioeroding organisms. Tropical bioerosion also produces significant amounts of fine-grained sediment, and it plays a role in carbon and calcium geochemical cycling. Because bioerosion is a rapid process closely tied to local environmental conditions, trace fossils of bioeroders can be used to unravel the history of relative sea level changes.

ROCKY SHORE BIOEROSION AND ENCRUSTATION AS A KEY TO ASSESSING SEA-LEVEL CHANGE: AN EXAMPLE FROM AN EMERGENT MID-HOLOCENE LIMESTONE COAST, LAGO ENRIQUILLO, THE DOMINICAN REPUBLIC

WILSON, Mark A., Dept. of Geology, The College of Wooster, Wooster, OH 44691, mwilson@acs.wooster.edu; CURRAN, H. Allen, Dept. of Geology, Smith College, Northampton, MA 01063; GREER, Lisa, Div. of Marine Geology and Geophysics, RSMAS, University of Miami, Miami, FL 33149.

Steep cliffs of massive Miocene limestone exposed along the north shore of Lago Enriquillo in the southwestern Dominican Republic provided substrates for a mid-Holocene rocky shore fauna. Lago Enriquillo is now a hypersaline lake, but during earlier Holocene time it was the site of a seaway connected to the Caribbean Sea. Fringing coral reefs flourished along its margins, and the submerged limestone cliffs were encrusted by bivalves, corals and serpulid worms. The subtidal zone of this rocky shoreline was also bored by bivalves, sponges and worms, producing Gastrochaenolites, Entobia and Caulostrepsis. The boring and physical erosion was particularly intense in the intertidal, forming deeply incised notches typical of exposed carbonates along tropical shorelines. The borings all belong to the deep-tier Entobia Ichnofacies of Bromley and Asgaard (1993), indicating that the bioerosion continued for many years without interruption. Hypersaline Lago Enriquillo formed when alluvial sediments and tectonic action closed off the opening of the seaway, trapping water which evaporated faster than it was replenished. The dropping levels of the lake have exposed the fossil coral reefs and rocky shore assemblages. Two littoral notches are visible in the emerged limestone cliffs, showing that there were two stillstands which lasted long enough to produce significant erosion in the intertidal. The uppermost notch is easily followed through most of the exposures, especially since it is topped by large serpulid/tufa mounds. The lower notch is discontinuous, probably because of variations in physical erosion. Nevertheless, the lower stillstand can be easily detected because the borings and encrusters show a distinctive distribution. In particular, an ichnological boundary between Caulostrepsis below and Gastrochaenolites and Caulostrepsis above maintains a constant elevation. The borings and encrusters show that sea-levels in the Enriquillo Seaway were relatively stable for at least two extended intervals. Sea-level changes between these intervals were rapid because there is little to no overlap in the paleontological assemblages.

"PSEUDOBRYOZOANS" AND THE PROBLEM OF ENCRUSTER DIVERSITY IN THE PALEOZOIC

Wilson, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691, USA.; and Paul D. Taylor, Department of Palaeontology, The Natural History Museum, London SW7 5BD, UK

Fossil marine communities that encrust hard substrates are important in our understanding of biodiversity through time. These organisms are often well-preserved, have distinct adaptive histories, and inhabited relatively consistent environments through the Phanerozoic. A problem, however, has been the erroneous or questionable assignment of some common Paleozoic runner-type encrusters to the bryozoans, thereby introducing errors into diversity estimates and paleoecological reconstructions. In a review of the evolutionary paleoecology of hard substrate communities, we have identified two such groups of "pseudobryozoans". The Suborder Hederelloidea comprise six genera of supposed cyclostome bryozoans ranging from the Silurian into the Permian. Hederelloids may not be bryozoans because: (1) their upper zooid size exceeds that known in bryozoans; (2) zooids are often budded from the sides of a broad stolonal tube; (3) the fibrous calcite wall structure is a fabric unknown in Paleozoic bryozoans; (4) zooids can be long, sinuous, prostrate tubes atypical of bryozoans. A second group of "pseudobryozoans" is represented by five encrusting genera (Allonema, Ascodictyon, Eliasopora, Vinella, Condranema) included in two families of ctenostome bryozoans in the Treatise (Bassler, 1953). However, these genera have calcified skeletons and thus are not ctenostomes. They consist of radiating clusters or ramifying chains of vesicles and/or narrow stolons. Some have a minutely porous skeleton, but none have apertures of sufficient size to permit the passage of a bryozoan lophophore. Removal of these "pseudobryozoans" reduces the diversity of encrusting bryozoans in the fossil record, especially in the Devonian, and leaves several unanswered questions about the true affinities and biology of these important components of Paleozoic hard substrate communities.

THE ORDOVICIAN BIOEROSION REVOLUTION

WILSON, Mark A., Department of Geology, The College of Wooster, Wooster, OH 44691, mwilson@acs.wooster.edu; PALMER, Timothy J., The Palaeontological Association, c/o Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion SY23 3DB, Wales, U.K.

It has become increasingly clear that the evolutionary radiation in the Ordovician included an extraordinary rise in bioeroders on carbonate hard substrates. Only one macroboring ichnogenus is known from the Cambrian, the simple, cylindrical Trypanites, and it is relatively rare and virtually confined to the Lower Cambrian. In contrast there are at least six macroboring ichnogenera in the Ordovician, and they are often important components of hard substrate communities. Trypanites reappears in abundance, and by the Late Ordovician the organisms producing it are recycling significant amounts of calcium carbonate. Palaeosabella is a similar macroboring with an expanded base; it shows a paleoecological distribution similar to that of Trypanites. Gastrochaenolites, a large clavate boring, is now known from Lower Ordovician hardgrounds in Sweden (Ekdale & Bromley, 2001, Lethaia 34: 1); its maker is unknown. Petroxestes is the facultative boring of mytilacean bivalves, and it is now known from the Middle and Upper Ordovician of North America. Ropalonaria represents ctenostome bryozoan etched borings especially common on calcitic shells of the Upper Ordovician. Cicatricula is a ramifying shallow boring found on Middle Ordovician hardground surfaces in Iowa; it appears to have been produced by some sort of boring sponge. These styles of bioerosion which appeared in the Ordovician remained dominant for the remainder of the Paleozoic. The only significant Paleozoic additions were acrothoracican barnacle borings in the Devonian, apparently bivalve-produced Gastrochaenolites in the Pennsylvanian, and the phoronid boring Talpina and the recurved worm boring Caulostrepsis, also in the Pennsylvanian. The boring communities do not significantly change after the Ordovician until the Mesozoic Marine Revolution brings a dramatic diversification in bivalve and sponge borings.