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Geology

Stratigraphy

stratigraphic illustrationThe Bear Gulch Limestone is one of a series of limestone lenses within the Upper Mississippian Heath Formation of Montana and North Dakota that have collectively been named the Bear Gulch Limestone Member. The Heath Formation, along with the underlying Otter and Kibbey Formations, are called the Big Snowy Group and are all considered Serpukhovian (Namurian E2b) in age. According to the 2004 Stratigraphic Congress on the Carboniferous, this puts the deposition of the Bear Gulch Limestone at about 323 million years ago. The Heath Formation consists of shallow marine and brackish water shales, linear channel sandstones, littoral to fresh-brackish water shales, limestone lenses, and supralittoral (above the tide-line) gypsum layers. The upper boundary of the Heath Formation in the outcrop area marks the Mississippian- Pennsylvanian boundary. Sands within the Heath and erosional features at its upper boundary indicate uplift from the south and a possible southern source of fresh water. Uplift to the south presages the ancestral Rocky Mountains.

Stratigraphic studies show that the Upper Mississippian was a period of brief epicontinental sea transgression in this region of western North America that culminated in a regression at the end of the Mississippian. The transgression resulted in the deposition of the Big Snowy Group in the Williston Basin and the narrow Central Montana Trough. This trough, which was intermittently active during the Upper Mississippian, isa geological feature extending from the Williston Basin in the east to the paleo-Pacific coast immediately west of the Montana-Idaho border. Surface geological mapping and oil well log data show a 160 km series of in-line limestone lenses within the Heath Formation that successively overlap one another from east to west, and demonstrate that the trough was subsiding intermittently from east to west during the deposition of the Heath Formation.

The Bear Gulch Limestone, being one of the uppermost, and apparently westernmost, of these lenses, developed during this time of both gradual and episodic subsidence. After being initially formed, this ever-shallow bay was filled in with muds and limes, leaving a maximum of 30 meters of Bear Gulch Limestone. The cumulative data on the deposition of the Bear Gulch Limestone suggests that it records a short lived (ca. one thousand years) marine environment that was subject to episodic tectonic activity superimposed on a trend of progressive infilling.

A general picture of the shape of the Bear Gulch basin shows a principal outlet on the northeastern corner, with the thickest part of the sequence (about 30 m of section) less than 1 km to the southwest. The principal basin axis extended westward for a minimum of 6 km. A geologic bar extended from the outlet southeastward. Both geologically-induced slope and flowindicators demonstrate a south-to-north slope immediately westward of the marginal bar. Further to the south, channels, slumps, and bar deposits strongly suggest a small separate tidal inlet through the eastern barrier. Overall, channel and slope directions converge upon the bay opening from the southwest, west, and west-northwest.

Boundaries

The uppermost layers of the Bear Gulch Limestone are characterized by stromatolites, mudcracks, disrupted beds, chert beds and nodules, dolomitic nodules, and local terrestrial plant material. These are succeeded in places by a thick conglomerate consisting of Bear Gulch clasts. Along its northern margin the Bear Gulch rapidly grades into thin dirty tan, barren, poorly consolidated marine silts and shales. A few of the upper layers along the northern margin contain considerable quantities of charcoal fragments, supporting the interpretation that these outcrops were very near shore and that the charcoal blew in from infrequenton-shore fires to the north. The eastern exposed margin includes an area of high-energy bay mouth facies fossils, foreset beds of a small prograding delta, dense brachiopod, sponge, and annelid worm zones, and a separate (probable tidal) inlet to open water marine conditions. The Bear Gulch Limestone grades into irregularly bedded dark grey to black shales basally.

Sediments

The Bear Gulch sediments are composed of very fine silts, lime silts, shell fragments, organic debris, and clays; silicates comprise up to 50 % by volume. Sedimentation in the central basin and nearby facies was in the form of rhythmically alternating sets of thick, hard, fine grained, massive to graded beds and sets of fine laminar beds; one unit may approach 1 meter in thickness. The thick, dense beds are sufficiently rich in decomposed plant matter and organic chemicals that they can be both tasted and smelled from freshly broken surfaces. There are no obvious traces of primary evaporites within the Bear Gulch.

Bear Gulch Limestone Map

Facies and Communities

The central basin facies is characterized by an almost total absence of sessile benthic invertebrates, very limited amounts of algae, and the highest diversity of fish and mobile and nectonic invertebrates. An Arborispongia-productid brachiopod facies is found along the northern and eastern margins as well as locally in the uppermost beds. It is characterized by dense stands of the arborescent sponge Arborispongia serving as the substrate for spiny articulate brachiopods, bivalve molluscs, and conulariids. South of the central basin facies lay a filamentous algal facies, characterized by high quantities of very fine filamentous algae in all lithologies accompanied by very small spiny productid brachiopods and a branched (colonial) species of Sphenothallus. The marginal facies is characterized by black, irregularly bedded to occasionally laminar bioturbated rocks. The acanthodian Acanthodes, commonly accepted as an indicator of brackish water conditions, is common in the western marginal beds.

The upper zone, or shallow facies of the Bear Gulch Limestone is characterized by whitish, tan to yellow beds, local relief surfaces with layers containing algal laminae and stromatolites, chert laminae and chert nodules. Cherts can imply fluviatile silica, in solution, being precipitated upon contact with marine water or shallow water. It can also imply diagenetic solution and redeposition of biogenic silica from sponge spicules and diatoms. Abundant Arborispongia communities, conulariids, large spiny productid and other articulate brachiopods, bivalved molluscs, stony bryozoans, some crinoids, and a branching species of Sphenothallus characterize the biota.

A limited variety of terrestrial leaves have been found in the uppermost layers, immediately below a capstone conglomerate composed of Bear Gulch Limestone fragments and a superposed thick marl zone. This represents the ultimate phase of shallowing and filling of the bay, starting from wider access to higher energy marine water and progressing to shoal conditions.

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References

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