Chapter 8: Paleoecology of the Cambrian in Montana and Wyoming

During Middle and Upper Cambrian time central and eastern Montana and all of Wyoming formed a stable shelf bordered on the west by a geosyncline (miogeosyncline) trending through western Montana, southern Idaho, and northeastern Utah. This foreland area, the drowned western coastal plain of Laurentia and Siouxia, experienced its initial downwarping during the early half of the Middle Cambrian with the shore line finally stabilized across eastern Montana and Wyoming by Elrathiellazone time. A second major downwarping during the Dresbachian stage carried the shore line eastward into the Upper Mississippi Valley region. During all this time sedimentation was slow, often negligible over large areas; shoals and low islands were prevalent, especially in south-central Montana and western Wyoming; and in both coastal and off-shore parts of the shelf extensive sand and mud flats were frequently exposed or awash. The preserved fossils are predominantly trilobites and inarticulate brachiopods but probably 75 per cent of the Cambrian biotas were soft-bodied plants and animals, The great number and variety of tracks, trails, and borings in the sediments is in-, triguing evidence of the presence of soft-bodied forms, and a few impressions of jellyfish, graptolites, and sponges have been obtained. Most of the assemblages represent thanatocoenoses, piles of trilobite molts, and broken brachiopod shells swept together and deposited by waves and currents. Only the assemblages of the algal meadows (Colleniareefs) and the Eoorthis-Billingsellabeds can be surely regarded as biocoenoses. The lithofacies as well as the fossils suggest, as might be expected over so large an area, that a variety of bottom conditions existed at any one time. Broad formational units of (1) a basal transgressive sandstone, (2) a succeeding shale, (3) a limestone with interbeds of shale, and (4) a regressive shale and limestone pebble conglomerate can be recognized for the Middle Cambrian over the entire region; in lithic detail each unit varies considerably. The basal sandstone may be missing; the limestone unit thins and wedges out in an easterly direction; and to the southeast, east, and northeast the entire section passes into a monotonous sequence of sands and shales carrying abundant evidences of shallow water deposition. During the earliest Upper Cambrian, conditions of sedimentation remained unchanged, and shales with intraformational pebble conglomerates constitute the lowest unit, succeeded by a fragmental oölitic limestone marking the major Dresbachian downwarping. The limestone wedges out eastward. The rest of the Upper Cambrian and basal Ordovician consists of a highly variable succession of sandy shales, calcareous shales, thin limestones, limestone pebble conglomerates, and dolomitic siltstones. The vertical and horizontal variability of these sediments indicates that the entire shelf area lay essentially at the level of equilibrium. No appreciable thickness of beds could be deposited, and the small amount of clastic material, the glauconite, and the limestone pebbles were reworked many times before final burial. From this lithic succession it is clear that the Middle-Upper Cambrian boundary as commonly recognized in this area is a facies boundary which is older in the west than in the east, rising in age across the shelf area; it does not correspond to the time boundary suggested by the fossils. The ecologic implications of the intraformational pebble conglomerates, the oölites, and the glauconite, all of which occur in striking abundance in the formations of the shelf area, are critically examined. Four types of pebble conglomerate are recognized, two of which are known to intergrade. Fragmentation by waves and currents is postulated; this may have occurred anywhere from exposed surfaces down to possibly 100 feet. Distinctions among the four types of conglomerate are based upon the amount of transportation after fragmentation and the probable depth of water in which final deposition occurred. Oölite is conspicuous in the fragmental limestones of Middle Cambrian and Dresbachian age and suggests deposition in a depth of 100 feet or less. Glauconite is absent from all transgressive sands or shales throughout the area, but it may appear directly above these beds and continue through the section becoming quite conspicuous in the higher beds of the Upper Cambrian. A compilation of information on glauconite permits these conclusions: (1) Several sites of autochthonous glauconite existed within this area. These sites are thought to have been broad swales, below the general level of the sea bottom, characterized by a very slow rate of sedimentation, a moderately anaerobic environment in the bottom sediments, and a large amount of putrefying organic matter. (2) For the sediments as a whole the presence of abundant glauconite suggests that they were derived from land areas where crystalline rocks were exposed and no large rivers reached the shore; the sea bottom supported a large and varied fauna, with a biota of scavengers and mud-eaters only in the broad swales. (3) The presence of glauconite in itself is not conclusive evidence of any definite water temperature or water depth. Five maps showing the general lithofacies distribution at five stages in Middle and Upper Cambrian time, and two paleogeographic maps are presented. A brief summary of early Ordovician depositional history and of post-Canadian pre-Upper Devonian tectonic and erosional histories is sketched in order to clarify the relations of the top of the Cambrian or basal Ordovician sequence of this area to the overlying beds.