Climatology
Global and Regional Paleogeography
Geomagnetic data place central Montana at about 10-12° north of the equator in Upper Mississippian time, or approximately at the present latitude of the southern Red Sea to the Sudan. The Visean-Namurian North American continent is reconstructed as rotated approximately 45° clockwise relative to its present orientation, with the continental plate moving north. In comparison to today's environments, and all other factors being analogous, Africa's Sahel would best approximate the climatic conditions of the Bear Gulch region; desert conditions are likely to have prevailed further north and more tropical conditions to the south. Because these latitudes are characterized by a monsoonal climatic regime of rainy and dry seasons, the Bear Gulch region is believed to have been subject to similar seasonal conditions resulting from shifts in the Intertropical Convergence Zone (ITCZ), the intensity of which would be defined by the magnitude of the ITCZ shift.
Paleoclimatology
When the ITCZ was
positioned closest to the paleo-equator the 10-12° latitude would
have been most likely to experience cool, dry east to northeasterly
winds during the winter season. Progression from the winter season into
spring and summer would have been defined by a northward shift in the
ITCZ. As the ITCZ increasingly encroached upon the latitude of the Bear
Gulch it would have introduced the increased probability of strong
westerly to southwesterly winds and precipitation, and a monsoonal-type
summer climate. Refreshment of the bay with epicontinental sea water
would have been extremely restricted in summer compared to winter
because wind-driven waters could only enter from a southeast channel
rather than the bay mouth proper and wind direction would counter the
impact of daily tidal flow.
Gypsum typically indicates arid conditions. Gypsum deposits in adjacent Heath Shale facies and gypsum and chert nodules in shallow facies corroborate the semi-arid to arid tropical aspect of the regional paleoclimate. Gypsum in the shallow facies and along the paleo-eastern margin represents periods of high evaporation rate during the hot and dry times as well as the terminal stages of filling in of the bay. There is an almost total absence of fresh-water macrofossils in the Bear Gulch Limestone. This is in spite of the narrowness of the Central Montana Trough and the proximity of the shores of the Bear Gulch bay; conditions that would have provided the shallow, marginal environments ideal to supporting freshwater communities if freshwater were regularly available. The alternative conditions of brackish and fresh water indicator fossils in the Heath paper shale, and the high percentage of the presumed brackish water indicator fish Acanthodes toward the western edge of the Bear Gulch lens, serve as seasonal and climatic counterpoints.
Microturbidites
are the dominant mode of sediment deposition in the Bear Gulch
Limestone. Either hypersalinity or fresher-more saline layering could
have provided an adequately dense bottom layer for the suspension of a
turbidity current composed of very fine particles and particle-organic
aggregates. Hypersalinity has previously been used as a mechanism to
explain the environment of deposition of the Heath Formation shales to
the east. Both conditions fit our seasonal model for the Bear Gulch.
Periods of aridity would result in warm, saline waters in the shallows
and a denser hypersaline layer of bottom water extending from the bay
head region to the zone of epicontinental sea infiltration. A storm or
other disturbance of the sediment during a period of density
stratification would result in a turbidity flow or detached turbidity
current along the pycnocline from the shallow paleo-western margins
eastward. Thus the available data support our proposal for seasons of
aridity and limited but torrential rainfall in a monsoonal climatic
regime. The possible effects of a stratified water column carrying an
organic-rich turbidity current into the central basin over a
hypersaline lower water layer are discussed below.
References
- Grogan, E.D., and R. Lund. 2002. "The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition." Geodiversitas (MNHN. Paris) 24:295-315.
- Witzke, B.J. 1990. "Palaeoclimatic constraints for Paleozoic Palaeolatitdes of Laurentia and Euramerica", in Mc Kerrow W.S. & Scotese C.R. (eds), Paleozoic palaeogeography and biogeography. Geological Society Memoirs 12: 57-73.
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