Abstract Small fossils are preserved as phosphatic (carbonate fluorapatite) micro-steinkerns (~ 0... more Abstract Small fossils are preserved as phosphatic (carbonate fluorapatite) micro-steinkerns (~ 0.5 mm diameter) in Upper Ordovician beds of the Cincinnati area. Mollusks are common, along with bryozoan zooecia, echinoderm ossicles, and other taxa. Similar occurrences of Ordovician micromorphic mollusks have been interpreted as ecologically dwarfed and adapted to oxygen-starved conditions, an interpretation with implications for ocean anoxia. An alternative explanation for small phosphatic steinkerns is taphonomic. Stable carbonate fluorapatite selectively filled small voids, thus preserving small fossils, including larval/young mollusks. Reworking concentrated small phosphatic steinkerns from multiple generations while larger, unfilled calcareous shells were destroyed, resulting in small fossils progressively replacing larger fossils. With thin sections and insoluble residues, we document evidence that many of these steinkerns are incomplete (“teilsteinkerns”) recording small parts of larger, normal-sized animals, or juveniles, along with smaller species. This finding suggests that these fossil assemblages are taphonomically, not ecologically, size-limited. Based on the ecology of modern oxygen minimum zones in which shelled mollusks are rare, the presence of abundant shelled organisms actually argues against severe oxygen stress. Our results also imply that the process by which the “small shelly fossils” of the Cambrian were preserved continued into the Ordovician.
Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone beddi... more Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone bedding surfaces in the Cincinnati Ordovician, are sometimes associated with moats, which are sediment depressions or gutters ~5-mm wide surrounding the commissure. Moats are interpreted as trace fossils, excavated by water expelled as the valves snapped shut. Other specimens vary from nearly horizontal to nearly vertical with the hinge line down and commissure up. Meniscate backfill beneath the anterior shell margin traces an arcing path formed as the shell rotated upwards around the posterior hinge line. Rotational tracks are interpreted as trace fossils, recording movement from an initial position buried horizontally to an inclined position as the brachiopod tried to escape burial. The traces form a continuum. Specimens lying flat on the bed surface have moats but no rotational tracks. Inclined shells are associated with deeper burial by obrution events and a greater arc of rotation. The moat shape is inconsistent with differential compaction. The precise association between moats and commissures and the independence of these structures on shell azimuth are inconsistent with current scour. If moats formed by rapid expulsion of water during valve snapping, then rotational tracks may have formed by a similar process. These traces are interpreted as fugichnia formed in response to catastrophic burial, but some moats could be equilibrichnia, formed by adjustment to minor sedimentary events. Rotational traces are similar to type 1 structures of Sowerbyella. If these two genera had similar tracemaking abilities, then other strophomenates probably shared these abilities.
Shell bed development can be a product of complex sedimentological and biological factors. The Up... more Shell bed development can be a product of complex sedimentological and biological factors. The Upper Ordovician sediments near Cincinnati, Ohio constitute a succession of thinly interbedded shelly carbonates and mudrocks. Despite years of study, the development of Cincinnatian shell beds and metre-scale cycles has, until recently, been attributed solely to storm reworking. This “storm-winnowing model” treats shells as passive sedimentary clasts, ignoring other factors of shell-bed development. A recently proposed alternative is Brett and Algeo’s idea that these shell beds grew during long periods of normally low sedimentation, while most mud accumulated during brief periods of high sedimentation. Under this “episodic starvation model”, any storms would winnow pre-existing muds and shell beds alike. We tested both models in the Edenian-Maysvillian (early to mid Katian) strata of the Cincinnati region by compiling observations on their petrologic, taphonomic, and paleoecological characteristics. The storm-winnowing does not explain several observed features that the episodic starvation model does, including: (i) storm-related sedimentary structures in mudrocks and limestones; (ii) lack of a sufficiently fossiliferous precursor deposit to winnow; (iii) deep-water faunas in grainstones; (iv) mixed taphonomic conditions of shell bed fossils; (v) ubiquitous discontinuity surfaces; (vi) carbonate concretion horizons; (vii) unwinnowed shell beds; and (viii) micrite in packstones. Episodic starvation is a superior explanation because it explains all of these features and allows for the complex interplay of other environmental and biological factors that contribute to shell bed growth. It may also be applicable to other deposits, previously interpreted as tempestites.
Like many Phanerozoic marine successions, the Upper Ordovician (Caradocian/Cincinnatian; Edenian)... more Like many Phanerozoic marine successions, the Upper Ordovician (Caradocian/Cincinnatian; Edenian) Kope Formation in southwestern Ohio and northern Kentucky exhibits distinct, correlatable alternations of thick (metre- to decametre-scale) mudrock-dominated intervals and thinner (decimetre-to metre-scale) shell bed-dominated units. The sedimentologicand taphonomic characteristics provide evidence for controls on the sedimentary and ecologic dynamics of the depositional system and differences in the relative amounts of time represented in mudrock- versus shell bed-dominant phases of the succession. Both phases record the episodic interruption of low-energy background sedimentation by highenergy events of storm-related scouring and reworking.
Evidence for event deposition in the Kope Formation is more apparent in strata rich in skeletal remains than those lacking such material. Due to their typically coarser grained nature compared to muds, shell-rich limestone beds commonly preserve features such as cross-bedding, grading, imbrication, rip-up clasts and bedforms. Conversely, evidence for storm activity, although detectable in mudrocks in some instances, is often subtle and less likely to be recognized than in bioclastic units due to their general lack of significant variation in mineralogy, grain size, colour, fabric and structure.
Our results also suggest that the thinner bioclastic units are condensed, relative to the mudrock-dominated intervals. The former represent the long-term accumulation of thin, mudstarved veneers of winnowed/reworked parautochthonous skeletal debris, whereas the latter involved the episodic deposition of mud layers up to several centimetres thick.
Three models have been proposed to explain the origin of the Kope mudrock–shell bed cycles. The first two assume that the mudrock intervals reflect low energy, deep-water conditions and that shell beds are primarily the product of increased storm wave energy and winnowing that, in turn, resulted from either relative sea level fall, during which storm wave-base was lowered closer to the seafloor, or increased intensity and/or frequency of storms without significant change in water depth. We envisage a third model in which the shell beds reflect not only storm winnowing, but also the accumulation of time-averaged skeletal debris during prolonged periods of siliciclastic sediment starvation, possibly associated with minor base-level rise.
Evidence presented herein indicates that the decimetre-scale shell beds of the Kope Formation formed during millennial-scale periods of siliciclastic sediment starvation combined with episodes of storm-related reworking and winnowing. This constitutes an alternative interpretation of shell bed genesis in the Kope Formation, as well as in many other mixed siliciclastic–carbonate successions, that is more in accord with taphonomic, sedimentologic and paleontologic evidence.
A uniquely preserved occurrence of the plectambonitoid brachiopod Sowerbyella rugosa from the Upp... more A uniquely preserved occurrence of the plectambonitoid brachiopod Sowerbyella rugosa from the Upper Ordovician of Northern Kentucky shows several individuals preserved in direct association with burrow-like structures and oriented with the commissural plane vertical and the hinge line down in the upper laminated siltstone of a tempestite bed. Two types of structures are present: (1) structures that document translational motion of convex-up brachiopods to the surface, then rotation to a vertical position with the commissure near the sediment-water interface; and (2) structures that show slightly longer-term occupation and sediment clearing around the brachiopods before they expired in the post-storm accumulation of fine mud. The abilities demonstrated by these individuals require a rethinking of the helpless-strophomenid paradigm. The peculiar plectambonitoid traits, mantle threads (as evidenced by interarea canals) and external mantle flaps (as evidenced by comae) were unlikely to have contributed to the ability of these individuals to burrow upward. Valve clapping is the only apparent means by which Sowerbyella rugosa could have moved through the sediment. If plectambonitoids could have moved by clapping their valves, then the same may have been true of strophomenoids, such as Rafinesquina alternata. The ability to move sediment and adjust position explains how strophomenids could live in a convex-up position or even, if partially buried, in more vertical positions, as documented by previous epibiont studies. These abilities suggest that geniculation may have served to anchor shells of active individuals in an inclined or vertical position in higher-energy environments of shifting substrates rather than to lift commissures of passive individuals in a convex-down position in lower-energy environments of rapid mud accumulation.
Environmental controls on the distribution of fossils most
commonly are found by recognizing that... more Environmental controls on the distribution of fossils most commonly are found by recognizing that certain distinctive fossil assemblages are associated with particular lithofacies. Lack of change in lithofacies commonly is used as indicating a lack of significant environmental effects on the stratigraphic distribution of fossils. The results presented here challenge that view. The Upper Ordovician Kope Formation of the Cincinnati, Ohio, area has long been considered a single unit, both lithostratigraphically and in terms of depositional environment. Gradient analysis of over 1000 fossil assemblages reveals subtle environmental control on the distribution of fossils, in the absence of obvious lithologic change. This gradient analysis is used to construct an ecological model of the Kope fauna, with values of preferred depth, depth tolerance, and peak abundance estimated for the most common fossils. This method, conducted within a single lithofacies, offers the potential for reconstructing sequence architecture because faunas can be more sensitive recorders of environment than lithofacies. In addition, the presence of subtle facies control as in the Kope raises the prospect that environmental controls on paleobiologic and biostratigraphic patterns may be more pervasive than generally acknowledged.
LETHAIA Dattilo, BF 1996 07 15: A quantitative paleoecological approach to high-resolution cyclic... more LETHAIA Dattilo, BF 1996 07 15: A quantitative paleoecological approach to high-resolution cyclic and event stratigraphy: the Upper Ordovician Miamitown Shale in the type Cincinnatian. Lethaia, Vol. 28, pp. 21-37. Oslo. ISSN 0024-1 164. Stratigraphic cycles ...
... Author(s). Benjamin F. Dattilo, Indiana University - Purdue University Fort Wayne. Document T... more ... Author(s). Benjamin F. Dattilo, Indiana University - Purdue University Fort Wayne. Document Type. Article. Publication Date. ... Opus Citation. Benjamin F. Dattilo (1993). The Lower Ordovician Fillmore Formation of western Utah: storm-dominated sedimentation on a passive margin. ...
Geological Society of America Field Guide 4 2003 Sequence stratigraphy of the Sank Sequence: 40th... more Geological Society of America Field Guide 4 2003 Sequence stratigraphy of the Sank Sequence: 40th anniversary field trip in western Utah Kevin R. Evans James F. Miller Department of Geography, Geology, and Planning, Southwest Missouri State University, Springfield, ...
Storm depositional processes have explained the origin of the interbedded limestone and shale tha... more Storm depositional processes have explained the origin of the interbedded limestone and shale that is characteristic of the Upper Ordovician in the Cincinnati region. The storm-winnowing model postulates that, under calm-water conditions, continuous mud deposition on a ...
Good exposures of the reefs occur discontinuously on the western slopes of the Arrow Canyon Range... more Good exposures of the reefs occur discontinuously on the western slopes of the Arrow Canyon Range. Uneven dolomitization obscures some reef structures while creating patches of well-preserved reef rock that are up to 15 m thick by 50 m wide. Some dolomitic intervals ...
Abstract Small fossils are preserved as phosphatic (carbonate fluorapatite) micro-steinkerns (~ 0... more Abstract Small fossils are preserved as phosphatic (carbonate fluorapatite) micro-steinkerns (~ 0.5 mm diameter) in Upper Ordovician beds of the Cincinnati area. Mollusks are common, along with bryozoan zooecia, echinoderm ossicles, and other taxa. Similar occurrences of Ordovician micromorphic mollusks have been interpreted as ecologically dwarfed and adapted to oxygen-starved conditions, an interpretation with implications for ocean anoxia. An alternative explanation for small phosphatic steinkerns is taphonomic. Stable carbonate fluorapatite selectively filled small voids, thus preserving small fossils, including larval/young mollusks. Reworking concentrated small phosphatic steinkerns from multiple generations while larger, unfilled calcareous shells were destroyed, resulting in small fossils progressively replacing larger fossils. With thin sections and insoluble residues, we document evidence that many of these steinkerns are incomplete (“teilsteinkerns”) recording small parts of larger, normal-sized animals, or juveniles, along with smaller species. This finding suggests that these fossil assemblages are taphonomically, not ecologically, size-limited. Based on the ecology of modern oxygen minimum zones in which shelled mollusks are rare, the presence of abundant shelled organisms actually argues against severe oxygen stress. Our results also imply that the process by which the “small shelly fossils” of the Cambrian were preserved continued into the Ordovician.
Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone beddi... more Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone bedding surfaces in the Cincinnati Ordovician, are sometimes associated with moats, which are sediment depressions or gutters ~5-mm wide surrounding the commissure. Moats are interpreted as trace fossils, excavated by water expelled as the valves snapped shut. Other specimens vary from nearly horizontal to nearly vertical with the hinge line down and commissure up. Meniscate backfill beneath the anterior shell margin traces an arcing path formed as the shell rotated upwards around the posterior hinge line. Rotational tracks are interpreted as trace fossils, recording movement from an initial position buried horizontally to an inclined position as the brachiopod tried to escape burial. The traces form a continuum. Specimens lying flat on the bed surface have moats but no rotational tracks. Inclined shells are associated with deeper burial by obrution events and a greater arc of rotation. The moat shape is inconsistent with differential compaction. The precise association between moats and commissures and the independence of these structures on shell azimuth are inconsistent with current scour. If moats formed by rapid expulsion of water during valve snapping, then rotational tracks may have formed by a similar process. These traces are interpreted as fugichnia formed in response to catastrophic burial, but some moats could be equilibrichnia, formed by adjustment to minor sedimentary events. Rotational traces are similar to type 1 structures of Sowerbyella. If these two genera had similar tracemaking abilities, then other strophomenates probably shared these abilities.
Shell bed development can be a product of complex sedimentological and biological factors. The Up... more Shell bed development can be a product of complex sedimentological and biological factors. The Upper Ordovician sediments near Cincinnati, Ohio constitute a succession of thinly interbedded shelly carbonates and mudrocks. Despite years of study, the development of Cincinnatian shell beds and metre-scale cycles has, until recently, been attributed solely to storm reworking. This “storm-winnowing model” treats shells as passive sedimentary clasts, ignoring other factors of shell-bed development. A recently proposed alternative is Brett and Algeo’s idea that these shell beds grew during long periods of normally low sedimentation, while most mud accumulated during brief periods of high sedimentation. Under this “episodic starvation model”, any storms would winnow pre-existing muds and shell beds alike. We tested both models in the Edenian-Maysvillian (early to mid Katian) strata of the Cincinnati region by compiling observations on their petrologic, taphonomic, and paleoecological characteristics. The storm-winnowing does not explain several observed features that the episodic starvation model does, including: (i) storm-related sedimentary structures in mudrocks and limestones; (ii) lack of a sufficiently fossiliferous precursor deposit to winnow; (iii) deep-water faunas in grainstones; (iv) mixed taphonomic conditions of shell bed fossils; (v) ubiquitous discontinuity surfaces; (vi) carbonate concretion horizons; (vii) unwinnowed shell beds; and (viii) micrite in packstones. Episodic starvation is a superior explanation because it explains all of these features and allows for the complex interplay of other environmental and biological factors that contribute to shell bed growth. It may also be applicable to other deposits, previously interpreted as tempestites.
Like many Phanerozoic marine successions, the Upper Ordovician (Caradocian/Cincinnatian; Edenian)... more Like many Phanerozoic marine successions, the Upper Ordovician (Caradocian/Cincinnatian; Edenian) Kope Formation in southwestern Ohio and northern Kentucky exhibits distinct, correlatable alternations of thick (metre- to decametre-scale) mudrock-dominated intervals and thinner (decimetre-to metre-scale) shell bed-dominated units. The sedimentologicand taphonomic characteristics provide evidence for controls on the sedimentary and ecologic dynamics of the depositional system and differences in the relative amounts of time represented in mudrock- versus shell bed-dominant phases of the succession. Both phases record the episodic interruption of low-energy background sedimentation by highenergy events of storm-related scouring and reworking.
Evidence for event deposition in the Kope Formation is more apparent in strata rich in skeletal remains than those lacking such material. Due to their typically coarser grained nature compared to muds, shell-rich limestone beds commonly preserve features such as cross-bedding, grading, imbrication, rip-up clasts and bedforms. Conversely, evidence for storm activity, although detectable in mudrocks in some instances, is often subtle and less likely to be recognized than in bioclastic units due to their general lack of significant variation in mineralogy, grain size, colour, fabric and structure.
Our results also suggest that the thinner bioclastic units are condensed, relative to the mudrock-dominated intervals. The former represent the long-term accumulation of thin, mudstarved veneers of winnowed/reworked parautochthonous skeletal debris, whereas the latter involved the episodic deposition of mud layers up to several centimetres thick.
Three models have been proposed to explain the origin of the Kope mudrock–shell bed cycles. The first two assume that the mudrock intervals reflect low energy, deep-water conditions and that shell beds are primarily the product of increased storm wave energy and winnowing that, in turn, resulted from either relative sea level fall, during which storm wave-base was lowered closer to the seafloor, or increased intensity and/or frequency of storms without significant change in water depth. We envisage a third model in which the shell beds reflect not only storm winnowing, but also the accumulation of time-averaged skeletal debris during prolonged periods of siliciclastic sediment starvation, possibly associated with minor base-level rise.
Evidence presented herein indicates that the decimetre-scale shell beds of the Kope Formation formed during millennial-scale periods of siliciclastic sediment starvation combined with episodes of storm-related reworking and winnowing. This constitutes an alternative interpretation of shell bed genesis in the Kope Formation, as well as in many other mixed siliciclastic–carbonate successions, that is more in accord with taphonomic, sedimentologic and paleontologic evidence.
A uniquely preserved occurrence of the plectambonitoid brachiopod Sowerbyella rugosa from the Upp... more A uniquely preserved occurrence of the plectambonitoid brachiopod Sowerbyella rugosa from the Upper Ordovician of Northern Kentucky shows several individuals preserved in direct association with burrow-like structures and oriented with the commissural plane vertical and the hinge line down in the upper laminated siltstone of a tempestite bed. Two types of structures are present: (1) structures that document translational motion of convex-up brachiopods to the surface, then rotation to a vertical position with the commissure near the sediment-water interface; and (2) structures that show slightly longer-term occupation and sediment clearing around the brachiopods before they expired in the post-storm accumulation of fine mud. The abilities demonstrated by these individuals require a rethinking of the helpless-strophomenid paradigm. The peculiar plectambonitoid traits, mantle threads (as evidenced by interarea canals) and external mantle flaps (as evidenced by comae) were unlikely to have contributed to the ability of these individuals to burrow upward. Valve clapping is the only apparent means by which Sowerbyella rugosa could have moved through the sediment. If plectambonitoids could have moved by clapping their valves, then the same may have been true of strophomenoids, such as Rafinesquina alternata. The ability to move sediment and adjust position explains how strophomenids could live in a convex-up position or even, if partially buried, in more vertical positions, as documented by previous epibiont studies. These abilities suggest that geniculation may have served to anchor shells of active individuals in an inclined or vertical position in higher-energy environments of shifting substrates rather than to lift commissures of passive individuals in a convex-down position in lower-energy environments of rapid mud accumulation.
Environmental controls on the distribution of fossils most
commonly are found by recognizing that... more Environmental controls on the distribution of fossils most commonly are found by recognizing that certain distinctive fossil assemblages are associated with particular lithofacies. Lack of change in lithofacies commonly is used as indicating a lack of significant environmental effects on the stratigraphic distribution of fossils. The results presented here challenge that view. The Upper Ordovician Kope Formation of the Cincinnati, Ohio, area has long been considered a single unit, both lithostratigraphically and in terms of depositional environment. Gradient analysis of over 1000 fossil assemblages reveals subtle environmental control on the distribution of fossils, in the absence of obvious lithologic change. This gradient analysis is used to construct an ecological model of the Kope fauna, with values of preferred depth, depth tolerance, and peak abundance estimated for the most common fossils. This method, conducted within a single lithofacies, offers the potential for reconstructing sequence architecture because faunas can be more sensitive recorders of environment than lithofacies. In addition, the presence of subtle facies control as in the Kope raises the prospect that environmental controls on paleobiologic and biostratigraphic patterns may be more pervasive than generally acknowledged.
LETHAIA Dattilo, BF 1996 07 15: A quantitative paleoecological approach to high-resolution cyclic... more LETHAIA Dattilo, BF 1996 07 15: A quantitative paleoecological approach to high-resolution cyclic and event stratigraphy: the Upper Ordovician Miamitown Shale in the type Cincinnatian. Lethaia, Vol. 28, pp. 21-37. Oslo. ISSN 0024-1 164. Stratigraphic cycles ...
... Author(s). Benjamin F. Dattilo, Indiana University - Purdue University Fort Wayne. Document T... more ... Author(s). Benjamin F. Dattilo, Indiana University - Purdue University Fort Wayne. Document Type. Article. Publication Date. ... Opus Citation. Benjamin F. Dattilo (1993). The Lower Ordovician Fillmore Formation of western Utah: storm-dominated sedimentation on a passive margin. ...
Geological Society of America Field Guide 4 2003 Sequence stratigraphy of the Sank Sequence: 40th... more Geological Society of America Field Guide 4 2003 Sequence stratigraphy of the Sank Sequence: 40th anniversary field trip in western Utah Kevin R. Evans James F. Miller Department of Geography, Geology, and Planning, Southwest Missouri State University, Springfield, ...
Storm depositional processes have explained the origin of the interbedded limestone and shale tha... more Storm depositional processes have explained the origin of the interbedded limestone and shale that is characteristic of the Upper Ordovician in the Cincinnati region. The storm-winnowing model postulates that, under calm-water conditions, continuous mud deposition on a ...
Good exposures of the reefs occur discontinuously on the western slopes of the Arrow Canyon Range... more Good exposures of the reefs occur discontinuously on the western slopes of the Arrow Canyon Range. Uneven dolomitization obscures some reef structures while creating patches of well-preserved reef rock that are up to 15 m thick by 50 m wide. Some dolomitic intervals ...
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in southwestern Ohio and northern Kentucky exhibits distinct, correlatable alternations of thick (metre- to decametre-scale) mudrock-dominated intervals and thinner (decimetre-to metre-scale) shell bed-dominated units. The sedimentologicand taphonomic characteristics provide evidence for controls on the sedimentary and ecologic dynamics of the depositional system and differences in the relative amounts of time represented in mudrock- versus shell bed-dominant phases of the succession. Both phases record the episodic interruption of low-energy background sedimentation by highenergy events of storm-related scouring and reworking.
Evidence for event deposition in the Kope Formation is more apparent in strata rich in skeletal remains than those lacking such material. Due to their typically coarser grained nature compared to muds, shell-rich limestone beds commonly preserve features such as cross-bedding, grading, imbrication, rip-up clasts and bedforms. Conversely, evidence for storm activity, although detectable in mudrocks in some instances, is often subtle and less likely to be recognized than in bioclastic units due to their general lack of significant variation in mineralogy, grain size, colour, fabric and structure.
Our results also suggest that the thinner bioclastic units are condensed, relative to the mudrock-dominated intervals. The former represent the long-term accumulation of thin, mudstarved veneers of winnowed/reworked parautochthonous
skeletal debris, whereas the latter involved the episodic deposition of mud layers up to several centimetres thick.
Three models have been proposed to explain the origin of the Kope mudrock–shell bed cycles. The first two assume that the mudrock intervals reflect low energy, deep-water conditions and that shell beds are primarily the product of increased storm wave energy and winnowing that, in turn, resulted from either relative sea level fall, during which storm wave-base was lowered closer to the seafloor, or increased intensity and/or frequency of storms without significant change in water depth. We envisage a third model in which the shell beds reflect not only storm winnowing, but also the accumulation of time-averaged skeletal debris during prolonged periods of siliciclastic sediment starvation, possibly
associated with minor base-level rise.
Evidence presented herein indicates that the decimetre-scale shell beds of the Kope Formation formed during millennial-scale periods of siliciclastic sediment starvation combined with episodes of storm-related reworking and winnowing. This constitutes an alternative interpretation of shell bed genesis in the Kope Formation, as well as in many other mixed siliciclastic–carbonate successions, that is more in accord with taphonomic, sedimentologic and paleontologic evidence.
commonly are found by recognizing that certain distinctive
fossil assemblages are associated with particular lithofacies.
Lack of change in lithofacies commonly is used as indicating
a lack of significant environmental effects on the
stratigraphic distribution of fossils. The results presented
here challenge that view. The Upper Ordovician Kope Formation
of the Cincinnati, Ohio, area has long been considered
a single unit, both lithostratigraphically and in terms
of depositional environment. Gradient analysis of over 1000
fossil assemblages reveals subtle environmental control on
the distribution of fossils, in the absence of obvious lithologic
change. This gradient analysis is used to construct an
ecological model of the Kope fauna, with values of preferred
depth, depth tolerance, and peak abundance estimated for
the most common fossils. This method, conducted within a
single lithofacies, offers the potential for reconstructing sequence
architecture because faunas can be more sensitive
recorders of environment than lithofacies. In addition, the
presence of subtle facies control as in the Kope raises the
prospect that environmental controls on paleobiologic and
biostratigraphic patterns may be more pervasive than generally
acknowledged.
in southwestern Ohio and northern Kentucky exhibits distinct, correlatable alternations of thick (metre- to decametre-scale) mudrock-dominated intervals and thinner (decimetre-to metre-scale) shell bed-dominated units. The sedimentologicand taphonomic characteristics provide evidence for controls on the sedimentary and ecologic dynamics of the depositional system and differences in the relative amounts of time represented in mudrock- versus shell bed-dominant phases of the succession. Both phases record the episodic interruption of low-energy background sedimentation by highenergy events of storm-related scouring and reworking.
Evidence for event deposition in the Kope Formation is more apparent in strata rich in skeletal remains than those lacking such material. Due to their typically coarser grained nature compared to muds, shell-rich limestone beds commonly preserve features such as cross-bedding, grading, imbrication, rip-up clasts and bedforms. Conversely, evidence for storm activity, although detectable in mudrocks in some instances, is often subtle and less likely to be recognized than in bioclastic units due to their general lack of significant variation in mineralogy, grain size, colour, fabric and structure.
Our results also suggest that the thinner bioclastic units are condensed, relative to the mudrock-dominated intervals. The former represent the long-term accumulation of thin, mudstarved veneers of winnowed/reworked parautochthonous
skeletal debris, whereas the latter involved the episodic deposition of mud layers up to several centimetres thick.
Three models have been proposed to explain the origin of the Kope mudrock–shell bed cycles. The first two assume that the mudrock intervals reflect low energy, deep-water conditions and that shell beds are primarily the product of increased storm wave energy and winnowing that, in turn, resulted from either relative sea level fall, during which storm wave-base was lowered closer to the seafloor, or increased intensity and/or frequency of storms without significant change in water depth. We envisage a third model in which the shell beds reflect not only storm winnowing, but also the accumulation of time-averaged skeletal debris during prolonged periods of siliciclastic sediment starvation, possibly
associated with minor base-level rise.
Evidence presented herein indicates that the decimetre-scale shell beds of the Kope Formation formed during millennial-scale periods of siliciclastic sediment starvation combined with episodes of storm-related reworking and winnowing. This constitutes an alternative interpretation of shell bed genesis in the Kope Formation, as well as in many other mixed siliciclastic–carbonate successions, that is more in accord with taphonomic, sedimentologic and paleontologic evidence.
commonly are found by recognizing that certain distinctive
fossil assemblages are associated with particular lithofacies.
Lack of change in lithofacies commonly is used as indicating
a lack of significant environmental effects on the
stratigraphic distribution of fossils. The results presented
here challenge that view. The Upper Ordovician Kope Formation
of the Cincinnati, Ohio, area has long been considered
a single unit, both lithostratigraphically and in terms
of depositional environment. Gradient analysis of over 1000
fossil assemblages reveals subtle environmental control on
the distribution of fossils, in the absence of obvious lithologic
change. This gradient analysis is used to construct an
ecological model of the Kope fauna, with values of preferred
depth, depth tolerance, and peak abundance estimated for
the most common fossils. This method, conducted within a
single lithofacies, offers the potential for reconstructing sequence
architecture because faunas can be more sensitive
recorders of environment than lithofacies. In addition, the
presence of subtle facies control as in the Kope raises the
prospect that environmental controls on paleobiologic and
biostratigraphic patterns may be more pervasive than generally
acknowledged.