Charles E. Savrda

Oxygen-related biofacies in marine strata : an overview and update

Abstract Recent studies of contemporary basins and ancient strata have provided a basis for expanding and refining early oxygen-related marine biofacies models. New observations permit higher resolution in palaeoecological, palaeoceanographic, and basin analyses. Five oxygen-related biofacies are currently recognized. (1) Anaerobic biofacies: well-laminated strata lacking in situ macro- and microbenthic body fossils and microbioturbation; may contain well-preserved remains of nektonic vertebrates, epiplanktonic or otherwise transported invertebrates, and faecal material of planktonic and/or nektonic origin. (2) Quasi-anaerobic biofacies: laminated strata, subtly disrupted by microbioturbation, containing microbenthic body fossils but lacking in situ macrobenthic body fossils; allochthonous body fossils and recognizable planktonic faecal material may be common. (3) Exaerobic biofacies: laminated strata similar to that of anaerobic or quasianaerobic biofacies but containing in situ epibenthic macroinvertebrate body fossils. (4) Dysaerobic biofacies: bioturbated strata characterized by low-diversity assemblages of relatively small, poorly calcified macrobenthic body fossils or absence of body fossils altogether. (5) Aerobic biofacies: bioturbated strata (where physical processes do not dominate) containing diverse assemblages of relatively large, heavily calcified macrobenthic body fossils. Recognition of these biofacies facilitates the recognition of relative palaeo-oxygenation gradients along the seafloor, vertically across the sediment-water interface, and through time. Application of sensitive trace-fossil models permits even more detailed reconstructions of benthic oxygenation histories for bioturbated strata, particularly those that fall within the previously defined dysaerobic realm.

Trace-fossil model for reconstructing oxygenation histories of ancient marine bottom waters: Application to upper cretaceous niobrara formation, Colorado

Abstract Trends of decreasing trace-fossil diversity, burrow size, and penetration depth with diminishing oxygen availability in contemporary marine settings have been incorporated with the concept of trace-fossil tiering to develop a model for reconstructing the oxygenation history of ancient basin bottom waters. More sensitive than traditionally-employed criteria, the model permits detailed evaluation of relative degree of oxygenation through the recognition of oxygen-related ichnocoenosis (ORI) units, or units of strata deposited under similar levels of bottom-water oxygenation. When employed in detailed vertical sequence analyses, the model may also be used to construct interpreted oxygenation curves that reflect rates and magnitudes of temporal change in redox conditions. The utility of the model is demonstrated by its application to rhythmically-bedded, calcareous strata of the Niobrara Formation (Upper Cretaceous) exposed in Colorado. Non-laminated strata of the Niobrara are characterized by four recurring trace-fossil suites referred to as the (1) Chondrites , (2) Zoophycos/Teichichnus , (3) Planolites , and (4) Thalassinoides assemblages. On the basis of trends in trace-fossil diversity, burrow diameter and penetration depth, these assemblages are recognized as oxygen-related ichnocoenoses. Interpreted oxygenation curves for the basal Fort Hays Member reflect high-magnitude redox cycles characterized by rapid deoxygenation events followed by gradual reoxygenation phases. Curves for the overlying Smoky Hill Member reflect an overall lower degree of oxygenation with more irregular, lower magnitude redox fluctuations. Significant correlation between interpreted levels of paleooxygenation and organic-carbon content illustrate the models potential for evaluating the distribution of organic-rich fine-grained strata. Co-interpretation of these variables with carbonate content supports the premise that deposition was controlled by paleoceanographic perturbations modulated by Milankovitch-like climatic cycles. Comparison with traditionally-emphasized macroinvertebrate body-fossil criteria clearly demonstrates the greater utility of the trace-fossil approach for evaluating paleo-oxygenation.

Ichnologic applications in paleoceanographic, paleoclimatic, and sea-level studies

Continued growth of ichnology over the last decade is reflected not only by an expanded realm of applications but also by the emergence of more holistic approaches involving the ichnofabric concept. Both of these trends are exemplified by recent ichnologic applications in paleoceanographic, paleoclimatologic, and sequence stratigraphic studies. Ichnofabrics and representative ichnocoenoses in pelagic/hemipelagic strata provide the basis for reconstructing benthic oxygenation histories of marine basins. Recent studies demonstrate that oxygen-related ichnocoenoses, when mapped within lithochronostratigraphic units, can help recognize spatial paleo-oxygenation gradients that reflect intrabasinal variations in circulation and govern organic matter accumulation. When cointerpreted with sedimentologic data, ichnology-based paleooxygenation histories also can be used to recognize and interpret paleoceanographic cycles and events, particularly those linked to climatic perturbations such as Milankovitch rhythms and El Nino-like phenomena. As demonstrated by recent investigations in the context of marine sequence stratigraphy, potential applications of ichnology in paleobathymetric studies extend well beyond the use of generalized ichnofacies models. Ichnofabric parameters can facilitate reconstructions of relative sea-level dynamics by providing supplementary or primary evidence used to delineate key sequence stratigraphic surfaces. Sequence boundaries and transgressive surfaces, which record erosional exhumation of underlying strata, typically are marked by well-expressed substrate-controlled ichnocoenoses (e.g., those representative of the firmground Glossifungites ichnofacies). Maximum flooding surfaces and other parasequence-bounding marine flooding surfaces can be recognized on the basis of abrupt to subtle vertical ichnofabric successions that signal sea-level-mediated paleoenvironmental change. INTRODUCTION

Ichnology in sequence stratigraphic studies; an example from the lower Paleocene of Alabama

Detailed ichno-sedimentological analyses of two Lower Paleocene depositional sequences contained within the Clayton and Porters Creek Formations of western Alabama. The ichnological variations reflect changes in substrate consistency, physical environmental energy, sedimentation rate, and bottom- and/or pore-water chemistry that are consistent with both the phases and magnitudes of the associated sea-level cycles. These results illustrate the potential of extended ichnological studies in the testing and refinement of sequence stratigraphic models and resulting interpretations of sea-level histories

Teredolites, wood substrates, and sea-level dynamics

Allochthonous logs and/or Teredolites , clavate borings produced within xylic (wood) substrates, occur in extraordinary abundance as sedimentary components in transgressive marine shelf deposits of the lower Paleocene Clayton Formation in western Alabama. It is pro-posed herein that the abundance and preservational state of these components were controlled by (1) an influx pulse of xylic substrates into marine and marginal-marine environments, (2) hydraulic concentration of substrates during ravinement, and (3) condensation associated with sediment starvation, all three of which are induced by sea-level rise. The proposed relations among fossil wood, ichnofossils produced therein, and sea-level dynamics may be of use in the discrimination of sequence stratigraphic packages and their bounding surfaces; these relations also have implications for paleobotanical prospecting and the biological evolution and stratigraphic distribution of marine organisms that inhabit xylic substrates.

Firmground Ichnofabrics in Deep-water Sequence Stratigraphy, Tertiary Clinoform-toe Deposits, New Jersey Slope

Abstract Sixteen erosional surfaces are recognized in a 144-m-thick condensed package of Tertiary (Eocene-Pliocene) clinoform-toe sediments recovered at ODP Site 1073 on the New Jersey slope. Most of these surfaces are associated with significant hiatuses or extremely condensed intervals defined by Sr isotopes or biostratigraphic data, and many can be linked to sequence boundaries defined in onshore and shelf seismic studies. All surfaces define the bases of fining upward sequences; they separate clay or biogenic muds below from authigenic glauconitic sandy muds or sands above. The entire Tertiary package is thoroughly bioturbated and dominated by ichnotaxa representing softground conditions. Burrow densities, burrow preservation, and the relative importance of certain ichnotaxa vary through the Tertiary package, reflecting changes in water depth, relative degree of condensation, and associated glaucony authigenesis, all related to margin progradation. Nonetheless, when individual sequences are considered, little or no change in softground ichnofossil assemblages is recognized across bounding surfaces. However, most surfaces are marked clearly by firmground Thalassinoides, burrow systems that penetrate deeply (up to 2 m) into subjacent clays and are characterized by extremely sharp walls and coarser glauconitic fills. In shallower shelf sequences, firmground ichnofabrics develop at sequence boundaries in response to subaerial exposure and transgressive ravinement. In contrast, the Tertiary firmgrounds on the New Jersey margin formed in deep water in response to phases of rapid transgression and net erosion; consolidated mud substrates were exhumed as a result of sediment starvation and bottom-current winnowing, facilitated by bioerosion, at or near the bases of slope clinoforms. These observations extend the previously established sequence stratigraphic utility of the substrate-controlled Glossifungites ichnofacies to deeper water facies.

Log-grounds and the ichnofossil Teredolites in transgressive deposits of the Clayton Formation (lower Paleocene), western Alabama

A thin (∼1.5 m) transgressive systems tract within the Lower Paleocene Clayton Formation of western Alabama, composed of, in ascending order, a sandy pebbly marlstone (transgressive lag), calcareous muds, and a chalk (condensed section), contains an unusual abundance of allochthonous logs with Teredolites, a clavate boring produced within xylic substrates. Borings are characterised by diameters ranging from 1 to 15 mm, high length/width ratios, contorted axes oriented primarily parallel to the grain of substrates, calcite linings, and rare terminal bioglyphs. Although no body fossil evidence for the affinity of the boring organisms is preserved, these morphological characteristics, which are most allied with the ichnotaxon T. longissimus, reflect the boring activities of wood-digesting bivalves of the family Teredinidae

Ichnology of rhythmically bedded Demopolis Chalk (Upper Cretaceous, Alabama); implications for paleoenvironment, depositional cycle origins, and tracemaker behavior

Ichnofossils typically are well expressed at bed transitions within rhythmically bedded marine sequences, owing to high contrast between ambient sediments and burrow fills derived from overlying layers. These piped zones can provide important information on paleoenvironments, paleoceanographic mechanisms responsible for depositional cyclicity, and tracemaker behavior. This is illustrated via quantitative analysis ofpiped-zone ichnofabrics at the transitions between chalk and marl beds within the Campanian Demopolis Chalk, western Alabama. The Demopolis Chalk is characterized by an ichnocoenosis dominated by Anconichnus, Chondrites, Planolites, Taenidium, Teichichnus, Thalassinoides, and Zoophycos. This ichnocoenosis reflects deposition in a quiet, well-oxygenated outer-shelf setting and indicates that redox and scour cycles were not important controls of carbonate cyclicity. A statistically significant positive relationship is observed between piped-zone burrow densities and carbonate contents of overlying strata. This relationship, when modeled in the context offluctuations in sedimentation rate and associated changes in the residence time of sediment in the zone of active bioturbation, is consistent with a clastic-dilution mechanism for carbonate cyclicity. The sediment fills of most piped-zone ichnofossils can be readily linked to ambient andlor overlying sediments. These linkages, combined with relationships among burrow densities, carbonate contents, and, indirectly, organic carbon contents of associated sediments, provide evidence for animal behavior. Such relationships in the Demopolis Chalk support the surface detritus feeder model for Zoophycos and invalidate previous interpretations of Taenidium and Teichichnus, at least for some pelagic carbonate substrates.

Ichnosedimentologic evidence for a noncatastrophic origin of Cretaceous-Tertiary boundary sands in Alabama

The Clayton sands, thin discontinuous sand bodies found at or near the Cretaceous-Tertiary (K-T) boundary at various sites in Alabama, previously have been attributed to (1) catastrophic tsunami deposition associated with a K-T boundary impact or, alternatively, (2) noncatastrophic transgressive infilling of incised valleys cut during a preceding sea-level low-stand. New observations on the geometry and ichnosedimentologic character of a Clayton sand body, enveloping strata, and associated bounding surfaces exposed along Mussel Creek (central Alabama) refute the tsunami origin and support the latter mechanism. Studies of other boundary deposits in the gulf region, which may similarly benefit from ichnofossil investigations, should consider more rigorously the evidence for sea-level change near the K-T transition.

Ichnology of fair-weather and storm deposits in an Upper Cretaceous estuary (Eutaw Formation, western Georgia, USA)

Abstract Exposures of the Eutaw Formation in western Georgia, eastern Gulf coastal plain, USA, were examined to evaluate the ichnologic signature of fair-weather and storm-related strata deposited within the central bay of a Late Cretaceous estuary. Ichnofabrics of fair-weather sandy muds and muddy sands compare favorably with those of previously well-documented estuarine facies of the Cretaceous Western Interior foreland basin. Trace fossil assemblages vary with sediment texture, reflecting proximity to sources and frequency of deposition of sands, but they generally are dominated by simple structures attributed to activities of trophic generalists (Terebellina, Teichichnus, and Planolites). Although this likely reflects environmental stress associated with estuarine dynamics, the degree to which salinity fluctuations governed trace fossil assemblages cannot be confidently assessed. Trace fossil assemblages in estuarine storm sands also are variable. Distal central bay tempestites include fair-weather ichnofossil suites. However, commonly in proximal parts of the central bay, rapid event-related accumulation of suspended clays immediately followed sand emplacement and thereby precluded burrowing of sands by subsequently established fair-weather organisms. By virtue of proximity to fluvial point sources and/or salinity gradients, this phenomenon may be unique to and diagnostic of certain estuarine settings. Ophiomorpha is nearly ubiquitous in storm sands. Normally attributed to post-storm colonization by opportunistic organisms, these burrow systems instead may represent the work of storm-transported crustaceans.