Shell Bed Formation Research Articles

Late Eocene marginal marine deposits and paleoenvironment characterisation from the Maadi Formation (Northern Eastern Egypt)

Two beds containing relatively thick and highly concentrated oyster shell banks occurring in the densely fossiliferous Wadi Al Abraq outcrop (Upper Eocene Maadi Formation, Cairo-Sukhna Road, Egypt). Such shell beds are almost monospecific. The upper level consists mainly of Ostrea clotbeyi Bellardi, topped by a considerable concentration of Carolia placunoides Cantraine, while in the lower level occurs Ostrea multicostata Deshayes. In both beds, the upper contact with the overlying marl bed (about 2 m thick) is sharp and sometimes erosive. Similarly, the contact with the underlying shale (1.5 m thick) is sharp and erosional. The shell beds are composed mainly of loosely packed oyster shells and their fragments (usually with a high proportion of disarticulated specimens) set in the sandy limestone matrix. The collected oysters show bioerosion and skeletobiont assemblages. Bioerosion trace fossils comprise ten ichnospecies while skeletozoans comprise five taxa. In general, shells of O. clotbeyi show a higher incidence of bioerosion than O. multicostata. The shells' chaotic orientation and their moderate fragmentation indicate that the shell bed formation was associated with high energy events. On the other hand, the low frequency of articulated specimens suggests that the shell beds are parautochthonous oyster banks' remnants. The taphonomic features of the studied assemblage indicate deposition in a shallow-water, wave-dominated environment.

Ecological snapshot of a population of Panopea within their traces (Pliocene, Agua Amarga subbasin, SE Spain)

Pliocene deposits cropping out in the Agua Amarga subbasin (Almería, SE Spain) include a composite shell bed made up of variously preserved and densely packed mollusks. The characteristics of the shell bed indicate deposition in shallow marine settings under a changeable sedimentation rate. The composite shell bed was formed through the amalgamation of several depositional events, mostly connected with storm events. During relatively slow sedimentation the bivalve Panopea colonized the substratum. The colonization took place over different stages of the shell bed formation, as reflected by differently preserved Panopea scattered throughout the shell bed. The upper part of the shell bed contains several tens of Panopea preserved within their burrows: Scalichnus cf. phiale Hanken et al., 2001. All bivalves in S. cf. phiale are articulated and preserved in life position. They are confined to a single horizon and most probably represent a single fossil population composed of adult individuals of Panopea resulting from anastrophic burial by storm deposits. As a consequence, the bivalves in their burrows succumbed at the same time; that is, the study case represents an “ecological snapshot” or ecological census.

Character of Shell Beds Flanking Herod Point Shoal, Southeastern Long Island Sound, New York

Abstract High biogenic productivity, strong tidal currents, shoal topography, and short transport distances combine to favor shell-bed formation along the lower flanks of a cape-associated shoal off Herod Point on Long Island, New York. This shell bed has a densely packed, clast-supported fabric composed largely of undegraded surf clam (Spisula solidissima) valves. It is widest along the central part of the western flank of the shoal where topographic gradients are steep and a stronger flood tide results in residual flow. The bed is narrower and thinner toward the landward margins where currents are too weak to transport larger valves and topographic gradients are gentle, limiting bed-load transport mechanisms by which the shells are concentrated. Reconnaissance mapping off Roanoke Point suggests that shell beds are also present at the other cape-associated shoals off northeastern Long Island, where relatively similar geomorphic and oceanographic conditions exist. These shell beds are important to the Lon...

Preservation of Autochthonous Shell Beds by Positive Feedback between Increased Hardpart‐Input Rates and Increased Sedimentation Rates

Abstract The preservation of nonrapidly buried autochthonous shell concentrations with noncementing epifaunal animals in life position presents a taphonomic dilemma if in fact an increase in shelliness is driven by a decrease in sedimentation rate. A 150‐cm‐thick, densely packed shell bed with brachiopods from the Lower Jurassic of Morocco shows lower levels of postmortem alteration than shell‐poor beds, indicating that its formation is primarily governed by variations in hardpart‐input rates. Varying dominance and size structure of the main shell producer, brachiopod Zeilleria rehmanni, indicate that its increased population density was the main trigger in the shell bed formation. Thinner and less common microbial crusts in the shell bed than in shell‐poor beds indicate that higher shelliness is not due to lack of sediment. On the basis of actualistic data from modern mussel and oyster shell beds, the suspension feeding of a high‐density population leads to high biodeposition rates through production of ...

The settling pattern of brachiopod shells: stratigraphic and taphonomic implications to shell bed formation and paleoecology

In order to understand the effects of shell morphology on the hydrodynamic behavior and potential for sorting by hydraulic processes, the settling velocities and falling patterns of Bouchardia rosea (Mawe) shells (Brachiopoda, Rhynchonelliformea) from the northern coast of Sao Paulo State, Brazil, were studied under laboratory and field conditions. B. rosea is a small, epifaunal, free-lying brachiopod with shells fabric similar to some forms of Paleozoic brachiopods. Bouchardiid-rich shell-beds are also common in many Tertiary (Miocene) rocks of South America. Twenty valves (10 dorsal and 10 ventral) and three closed articulated shells, encompassing the typical size range of B. rosea were selected to the experiment. Shells were dried and weighed in air to the nearest 0.01g. The diameters of the three major axes of shells were measured to the nearest 1mm. Hydraulic settling velocities were recorded by releasing shells in a 35 cm wide and 56 cm deep column filled with natural salt water. Settling times were measured to the nearest 0.01 second, but falling durations were rounded to the nearest 0.1 second for calculation of settling velocities. Each shell was released just beneath the water surface for ten consecutive runs (230 releases total). The average settling velocities is 15.35 cm/s for ventral valves and 16.65 cm/s for dorsal ones. Articulated shells have average settling velocities of 26.48 cm/s. The lower average settling velocities of ventral shells indicate that they can be transported to greater distances than the less convex dorsal valves. This may be explained, in part, by the fact that ventral valves have greater cross-sectional areas than dorsal ones of the same weight. Hence, differential fragmentation of valves is not the only process likely to produce such bias, observed in some modern beach deposits, and many fossil concentrations. Finally, disarticulated shells settle in a concave-up orientation, regardless the size or degree of concavity of shells. Hence, biofabrics with shells in a preferred concave-up orientation will be common in fossil concentrations generated by turbulent flows with suspension settling of particles.

Not armour, but biomechanics, ecological opportunity and increased fecundity as keys to the origin and expansion of the mineralized benthic metazoan fauna

This paper offers a new biotic interaction hypothesis for the Cambrian ‘explosion’ of mineralized, benthic, metazoan diversity. It proposes that organic-mineral composite structures (e.g. shells and muscle lever-arms) originated in Proterozoic lineages of primary larva-like, but reproductively competent, pelagic bilaterians because mineralization was both mechanically and energetically favourable, not because it provided armour against predation. Increased strength and rigidity of composite structures permitted growth to sizes incompatible with a continued pelagic existence, while the increased density resulting from massive mineralization facilitated settlement into, and stability in, a nutrient-rich, Proterozoic benthic zone that offered new ecological opportunities. Because evolutionary success is recognized by the formation of recoverable fossils, which requires large, enduring populations, successful lineages are those that responded to the new opportunities by achieving broad niche occupancy through the evolution of metamorphosis to larger, mineralized ‘adult’ body forms with more efficient food-collecting apparatus and higher fecundity. Niche modification (e.g. reef and shell-bed formation) by early mineralized benthic settlers may have increased the likelihood of further successful settlement, leading to the appearance of a period of ‘explosive’ increase in benthic, mineralized, metazoan diversity. Predator-prey arms races may then have followed, causing early faunal turnover and possible selection for improved armour. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 483‐490. ADDITIONAL KEYWORDS: benthic settlement - Cambrian explosion ‐ metamorphosis - organic-mineral composite.

Taphonomy and paleoenvironment of two turritellid‐gastropod‐rich beds, Pliocene of Florida

Two beds containing large numbers of turritellid gastropods (Family Turritellidae) occurring in the densely fossiliferous Upper Pliocene Pinecrest Sand of Florida formed as a result of upwelling and consequent high biological productivity, together with some degree of physically mediated time averaging. Analyses of size-frequency distribution and shell surface condition, combined with isotopic data on chronological age of individual shells, water temperature and upwelling intensity, suggest that both beds formed relatively quickly, probably in less than 100–200 years. The upper bed, occurring within Petuch's (1982) unit 2 (2.5-2.0 Ma) and containing abundant Turritella apicalis Heilprin, appears to have formed largely as a result of upwelling; the lower bed, occurring in upper unit 6/7 (3.5-2.5 Ma) and containing abundant Turritella gladeensis Mansfield, appears to have formed over a longer period, as a result of upwelling, increased time-averaging, and perhaps cooler overall water temperatures. This study highlights the potential to isolate and examine separately some of the biological and physical factors affecting shell bed formation, and especially to address the role of biological productivity in this process. □Pinecrest Beds, Pliocene, upwelling, turritellid gastropods, taphonomy.

Age, Environment and Mode of Deposition of the Densely Fossiliferous Pinecrest Sand (Pliocene of Florida): Implications for the Role of Biological Productivity in Shell Bed Formation

The «Pinecrest Sand,» an abundantly fossiliferous Late Pliocene deposit best exposed near Sarasota, Florida, is a poorly understood stratigraphic unit of considerable paleobiological interest because of its geographic and temporal position and extremely abundant and diverse mollusk fauna. The absolute age of this unit has been a subject of some debate, and has considerable implications for its taphonomic history. Evaluation of published and unpublished data indicates that the Pinecrest at Sarasota spans more than one million years, the lower beds having been deposited between 3.5 and 3.0 Ma and the upper beds between 2.5 and 2.0 Ma

Geographic gradients in the formation of shell concentrations: Plio-pleistocene marine deposits, Gulf of California

Shell bed (coarse concentrations of invertebrate bioclastic material) are rich sources of paleontologic and stratigraphic information. Studies often emphasize the relationship between shell bed formation and bathymetric gradients, because many sedimentologic and ecologic factors that influence shell-concentrating processes vary with water depth. However, shell bed formation is also influenced by factors that are not related to water depth. Shell beds in Plio-Pleistocene marine sediments of the Gulf of California (Mexico) formed in less than 10–15 m water depth, but under varying conditions of coastal topographic relief and tidal range. Sampling of shell beds focused on two regions: the northern mainland coast and the southern peninsular coast. Shell beds in the northern mainland region (13 total) formed along a macrotidal, low-relief coastline, whereas shell beds in the southern peninsular region (27 total) formed along a microtidal, high-relief coastline. Shell beds represent community beds (11); storm beds (3); beach berm beds (6); tidal channel bed (1); or current/wave-winnowed beds (19). The community beds represent biogenic concentrations; the other four types represent sedimentologic concentrations. Sedimentologic concentrations formed either through short-term concentrating events (storm beds), or long-term dynamic bypassing (beach berm beds, tidal channel beds, and current/wave-winnowed beds). Concentrations formed through dynamic bypassing are far more common (26 beds) than storm concentrations (3 beds). Of the 26 concentrations formed through dynamic bypassing, 10 cap major stratigraphic discontinuities, representing temporal breaks ranging from 10 3 to 10 8 years. The abundance and distribution of the five types of shell beds is significantly different between the two sampling areas. The northern mainland coast is dominated by allochthonous beach berm beds capping linear beach ridges. The formation of beach ridges in the north is favored by the wide shallow shelf (representing a large reservoir for beach shells), and sediment starved conditions resulting from the low topographic relief and periodic reductions in sediment supply from the Colorado River delta. Beach berm beds are absent along the southern peninsular coast, where coarse terrigenous sediment shed from adjacent highlands overwhelms shell input to the beach, and the narrow steep shelf supplies the beach with few shells. The southern peninsular coast is dominated by parautochthonous current/wave-winnowed beds and autochthonous community beds. Southern shell beds have undergone significantly less reworking, winnowing and transportation than northern shell beds. This is a function of both the relative weakness of tidal currents in the south, and the relatively higher rates of terrigenous sedimentation along the rugged southern coastline, which can bury shell concentrations relatively rapidly. Geographic variation in topographic relief and tidal range results in significant variation in the distribution and abundance of shell beds within these shallow marine sediments. These variables have root tectonic cause (relief reflects tectonic activity; tidal range reflects basin configuration), underscoring the potential utility of fossil concentrations for reconstructing earth history.

Biological and Physical Contributions to the Accumulation of Strombid Gastropods in a Pliocene Shell Bed

Before the evolutionary and ecological information contained in shell beds can be interpreted, the conditions of shell bed formation must be understood. Here the authors investigate the mode and time scale of accumulation of a dense layer of Strombus floridanus in the Pliocene Pinecrest Beds of Florida. They utilize a variety of comparative taphonomic data, including the extent of encrustation and boring on strombid shells of different ontogenetic ages, and on accompanying pelecypods of different ecological types. The taphonomic comparisons enable them to reconstruct more accurately the events of shell bed formation. The formation of the strombid shell layer involved both biological and physical components. The characteristically gregarious behavior of Strombus is reflected in the large number of individuals preserved in this layer. Based on average densities of individuals in strombid colonies today, the authors estimate that a time period of tens to hundreds of years was required to accumulate these fossils. Repeated sediment winnowing by storms, followed by rapid reburial in a regime of at least episodically high sedimentation rates, is the most likely mechanism of accumulation, and accounts for the observed patterns of encrustation and boring on the shells of Strombus and various associated pelecypods. The origin ofmore » Florida's Plio-Pleistocene shelly sands is poorly understood; analysis of this bed may provide a working model for future taphonomic studies.« less

Biogenic and Physical Modes of Stratification and Shell Bed Formation in the Recent Intertidal Environments of Bahia la Choya, Northern Gulf of California

Fossil concentrations such as shell beds are preferred collection sites for paleontological and paleoecological data. Although ample attention has been paid to the roles and implications of physical processes in shell bed formation, the role of biogenic sediment reworking in generating shell beds has received little study.Subsurface shell beds in the Recent intertidal environment at Bahia la Choya form via three distinct processes: storm deposition, tidal channel migration, and biogenic reworking. Shell beds on the outer tidal flats are produced primarily by storm sedimentation. Lateral migration of the main tidal channel produces shell beds along the channel margins. Shell beds on the inner flats distant from the main tidal channel are generated by vertical “conveyorbelt” recycling of fine sediment by deposit feeders. Biogenic reworking concentrates shells at depth into subsurface shell beds, producing a distinct form of biogenic bedding, termed biogenic stratification.These three modes of shell bed formation leave recognizable taphonomic and sedimentologic signatures. Biogenic stratification in particular has important implications for sedimentation rates, sediment reworking, and faunal condensation in ancient strata.