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Deposition

The sedimentation rate in the Bear Gulch Limestone must have been extremely rapid to account for the extraordinary preservation of the fossils. However, there is no extensive scouring of sediment surfaces and their organic drapes, so some mechanism of sediment transport other than bottom currents must be found. Furthermore, a rapid sedimentation rate alone will not account for the observation that many aspects of the fossil preservation demand simultaneous or near-simultaneous death and burial in real, not geological, time. These same aspects of preservation all but eliminate the possibilities of postmortem transport. The pervasive benthic fish faunal component excludes any possibility of persistent anoxic bottom water or persistent anoxic sediment conditions in the central basin axis. Therefore, it is believed that a combination of sedimentary and hydrological conditions must have been factors here.

rock

Multiple microturbidite layers in a polished section of Bear Gulch limestone. Note the dewatering faults (D).

Only detached turbidity currents would account for sudden, rapid influx of high sediment density on the scale necessary to trap and bury local inhabitants of the bay, the significant kill horizons, and the sedimentological features of these sites. These sediment-rich flows would have been carried out into the basin and deposited from above without scouring either the previously deposited laminae or their organic drapes. Several conditions must be met to explain the development of the rhythmicity of microturbidite deposition in the Bear Gulch bay.

Regular episodes of density stratification depend upon regular episodes in which water mixing does not occur. This repetition must have occurred constantly over the interval of geological time represented by the Bear Gulch lithology and the only factor that explains these conditions is seasonal reversal of wind and circulation patterns in the bay. The necessary seasonal conditions would probably be warm and dry, resulting in an evaporation rate in the bay considerably in excess of the rate of replenishment by lower salinity water.

The absence of water turnover or mixing would also depend upon little or no strong winds, or a prevailing wind direction that would not drive lower salinity water from the eastern or southern seaways into the bay. The known geology of the Bear Gulch Limestone and associated facies strongly suggests that the lip or bar at the mouth of the Bear Gulch bay would restrict refreshment of the bay water under easterly or southeasterly wind conditions.

Winds are the most relatively predictable and normal occurrences in any monsoonal climate that could resuspend quantities of fine accumulated shoreward sediments. Resuspended sediments would travel downslope toward the basin center. In the presence of density stratification a sediment-rich turbidity flow would travel on top of a pycnocline until the energy of the flow was exhausted and settling occurred. Episodic monsoonal rainfall would also generate these conditions as runoff carried sediment from the recently emergent land surrounding the Bear Gulch bay into the warm and hypersaline waters of the shallow margins and bay head.

The known direction of sediment transport was transverse to the bay, essentially from paleo-southwest to northeast. This agrees with transport due to geostrophic flow. Moreover, considerable quantities of sediment had to be accumulated at the margin of this small bay, and high quantities had to be transported in each event, to generate the thickness of individual laminae seen in the Bear Gulch Limestone. Detached turbidity currents also meet these criteria.

The high amounts of plant debris, algal mucus and organic compounds known to be contained in the sediments transported from the bay margin would contribute significantly to the rapid death of organisms trapped below a turbidity flow. The high salinity, density stratified bottom water layers could quickly become lethal as organic -rich debris cascaded downward from the upper water column and absorbed dissolved oxygen in the lower water column. The resultant combination of high salinity, high temperature, and oxygen depletion would be lethal within minutes to any living thing trapped in the lower part of the water column as sediments settled.

Rapid restoration of normal water column mixing and thus normal hydrological conditions when the wind shifted would result in speedy biological re-exploitation of the deeper waters from the periphery of the bay as well as from external opportunistic species. However, the regular high turbidity and high rate of fine sediment deposition, as well as the regular cutoff of replenishing bottom water inflow from the epicontinental sea could very well have prevented the establishment or long term survival of a sessile benthic biota.

The data, therefore, support an annual model for deposition of the rhythmic sedimentary units of the Bear Gulch Limestone, with intraseasonal intervals of density stratification providing the only known mechanism to suspend turbidity flows so regularly throughout the history of the Bear Gulch Bay.

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References

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