Microfossil bonebeds are multi-individual accumulations of disarticulated and dissociated vertebrate hardparts dominated by elements in the millimeter to centimeter size range (≤75% of bioclasts ≤5 cm maximum dimension). Modes of accumulation are often difficult to decipher from reports in the literature, although predatory (scatological) and fluvial/hydraulic origins are typically proposed. We studied the sedimentology and taphonomy of 27 microfossil bonebeds in the Campanian Judith River Formation of Montana in order to reconstruct formative histories. Sixteen of the bonebeds examined are hosted by fine-grained facies that accumulated in low-energy aquatic settings (pond/lake microfossil bonebeds). Eleven of the bonebeds are embedded in sandstones that accumulated in ancient fluvial settings (channel-hosted microfossil bonebeds). In lieu of invoking separate pathways to accumulation based on facies distinctions, we present a model that links the accumulation of bioclasts in the two facies. We propose that vertebrate material initially accumulates to fossiliferous levels in ponds/lakes and is later reworked and redeposited as channel-hosted assemblages. This interpretation is grounded in reasonable expectations of lacustrine and fluvial depositional systems and supported by taphonomic data. Moreover, it is consistent with faunal data that indicate that channel-hosted assemblages and pond/lake assemblages are similar with regard to presence/absence and rank-order abundance of taxa.This revised model of bonebed formation has significant implications for studies of vertebrate paleoecology that hinge on analyses of faunal data recovered from vertebrate microfossil assemblages. Pond/lake microfossil bonebeds in the Judith River record are preserved in situ at the scale of the local paleoenvironment, with no indication of postmortem transport into or out of the life habitat. Moreover, they are time-averaged samples of their source communities, which increases the likelihood of capturing both ecologically abundant species and more rare or transient members of the paleocommunity. These attributes make pond/lake microfossil bonebeds excellent targets for paleoecological studies that seek to reconstruct overall community membership and structure. In contrast, channel-hosted microfossil bonebeds in the Judith River record are out of place from a paleoenvironmental perspective because they are reworked from preexisting pond/lake assemblages and redeposited in younger channel facies. However, despite a history of exhumation and redeposition, channel-hosted microfossil bonebeds are preserved in relatively close spatial proximity to original source beds. This taphonomic reconstruction is counter to the commonly held view that microfossil bonebeds are biased samples that have experienced long-distance transport and significant hydrodynamic sorting.

Over Phanerozoic time scales, stratigraphic records from continental interiors, or cratons, are dramatically thinner and are generally assumed to be relatively incomplete, with more numerous and longer-duration hiatuses, compared to records from more rapidly subsiding continental margins. However, this assumption need not be true for the shorter time scales (i.e., several 10 6 years and shorter) over which accumulation of the preserved stratigraphic record on the continental interior actually takes place. This study compares carbonate-dominated sedimentary records from the Devonian continental interior (Iowa) and continental margin (Nevada) to evaluate whether the continental-interior record is (1) miniaturized, i.e., thinner, but equally complete; (2) comparable in thickness and quality where the sedimentary record is preserved, but certain portions of the record are notably absent, i.e., omitted or truncated; or (3) so invariably preserved that equivalent stratigraphic packages cannot be recognized compared to the continental-margin record.This study finds that fourth-to fifth-order cycles in Iowa are, on average, approximately half as thick and half as numerous compared to the continental margin of Nevada. Moreover, the stratigraphic record on the continental margin of Nevada is dominated by cycles that are composed of tidal-flat and shallow subtidal lithofacies, capped by simple flooding surfaces, and exhibit catch-up or keep-up depositional styles. The continental-interior record in Iowa is dominated by cycles that are composed of subtidal lithofacies, capped by a range of bounding surfaces, and exhibit catch-down or give-up depositional styles. These findings suggest that the thin record in Iowa is largely a result of low sedimentation rates associated with suppressed carbonate production in this epeiric-sea setting, rather than minimal subsidence rates and an increased potential for subaerial exposure and associated loss of record.

Discontinuity surfaces develop in carbonate successions in response to a range of environmental changes and represent an integral part of the stratigraphic record. In Palaeozoic shallow epeiric basins that are typified by extremely slow subsidence and intermittent sedimentation, discontinuity surfaces may represent the majority of the time-rock record. A depositional and sequence-stratigraphic model was developed through microfacies analysis and discontinuity surface characterization using three cores in a proximal to distal transect across the Middle to Upper Devonian Iowa Basin. Twelve microfacies are recognized, spanning supratidal to deep subtidal facies tracts. A total of 105 discontinuity surfaces were documented and classified as either submarine omission surfaces, subaerial exposure surfaces or submarine erosional surfaces. Omission surfaces increase in frequency basinward, indicating increased sediment starvation in the offshore direction. Exposure surfaces increase in frequency shoreward, indicating more frequent subaerial exposure in a shallower setting. Erosional surfaces are dominant in the inner and middle ramp and interpreted as the base of storm beds (tempestites); these surfaces are rare in the outer ramp due to its generally deeper setting below storm wave base. Moreover, discontinuity surfaces exhibit systematic groupings stratigraphically (vertically) across the three localities spanning the Devonian carbonate ramp. Zones of either exposuredominated, erosion-dominated or omission-dominated surfaces were recognized and correlated with their landward or basinward equivalents (along with shifts in major facies belts) and interpreted in a sequence-stratigraphic context. This study highlights the importance of including a detailed characterization of both depositional facies and non-depositional discontinuity surfaces in order to better understand the stratigraphic history of a basin. The framework of analysis provided here is particularly useful for marine carbonate strata deposited in epeiric basins, which are especially common in the Palaeozoic and where nondeposition and erosion occur frequently, but can also be applied to other geological time periods and settings.

Skeletal concentrations, defined here as deposits ! 10% by volume invertebrate bioclasts (. 2 mm), are very common targets of paleobiological investigations. The complex interactions among biological, taphonomic, and physical environmental processes influence the type and quality of information that can be drawn from these paleobiological repositories. This study examines the relative roles of bioclast input and burial on the formation of Middle-Upper Devonian skeletal concentrations from tropical carbonate-dominated settings. Based on original field observations of skeletal concentrations and host lithologies, skeletal concentrations are compared (1) between a thermally subsiding passive margin (Nevada) and a relatively stable cratonic interior (Iowa) and (2) across a range of comparable subtidal depositional environments. The majority of skeletal concentrations are thin ( 30 cm), monotypic, have simple internal stratigraphy, and exhibit moderate to high degrees of skeletal fragmentation and disarticulation. Compared to the subsiding margin of Nevada, the cratonic interior of Iowa preserves a higher proportion of polytaxic skeletal concentrations with complex internal stratigraphy and higher taphonomic damage, consistent with control by delayed or slow burial. From shallow to deep subtidal environments, stratigraphic thickness and internal complexity decrease and fragmentation increases, consistent with strong control of these attributes by the frequency of physical and biogenic reworking. While low bioclast production rates may exert a dominant control on the failure to accumulate thick, dense skeletal concentrations in these deposits, taphonomic and physical environmental processes, such as low net sediment accumulation, sediment winnowing and starvation, and bioturbation can explain much of the variation in Middle-Upper Devonian skeletal concentrations.

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