Abstract

The Morrison, Cloverly, and Sykes Mountain formations are the uppermost Jurassic and lowermost Cretaceous sedimentary rocks in the northern Bighorn Basin, Wyoming and Montana. Similar formations were deposited contemporaneously throughout most of the Western Interior.

Most studies of nonmarine rocks have been in localities of more rapid sedimentation, whereas the rocks of this study accumulated slowly, under virtually atectonic conditions.

The Morrison formation of the Bighorn Basin, as here restricted, includes the earliest-formed nonmarine sedimentary rocks of this sequence, conformably overlying the marine Sundance formation. The rocks are interlensed calcareous quartz sandstones, green mudstones and shales, and subordinate limestones, with locally conspicuous red-banded mudstones.

Overlying the Morrison formation, generally with conformity, is the Cloverly formation, as redefined in this report. The lowest of its three members, here named the Little Sheep mudstone members, consists chiefly of bentonitic (montmorillonitic) mudstones in variegated shades of neutral gray, purple, olive, and dusky to pale red. Other typical lithologies include bentonites, cherts, coaly beds, calcareous nodules, and chert-pebble conglomeratic sandstones. The Pryor conglomerate member in the northernmost Bighorn Basin is characterized by black-chert pebbles and rests unconformably on the Morrison formation. Its beds are the stratigraphic equivalent of the mid-Little Sheep conglomeratic sandstones farther south. The upper member of the Cloverly formation, here named the Himes member, comprises three principal lithologies. Commonly at its base is olive-gray and reddish-brown clay-matrixed salt-and-pepper sandstone. Most of the member is variegated reddish- and yellowish-brown and gray kaolinitic claystone and mudstone, containing veinlets and hardpans of iron oxides. Clean quartz sandstones which filled fluvial channels are laced through the claystones.

Disconformably overlying the Himes member are sandstones and thinly interbedded, rusty-brown–weathering siltstones, dark shales, and ironstones, here named the Sykes Mountain formation. The Sykes Mountain formation grades into the overlying marine Thermopolis shale.

Distinction of stratigraphic units on a lithogenetic basis is believed to eliminate the confusion which existed in previous nomenclature of this sequence.

Characteristic primary and secondary structures, clay, accessory, and authigenic minerals, and gross stratigraphic distribution aided the interpretation of the origin and history of these deposits. The Morrison formation accumulated in fluvial, lacustrine, and flood-plain environments from detritus derived chiefly from erosion of sedimentary rocks west of the present Bighorn Basin. The Little Sheep mudstone member and most of the Himes member of the Cloverly formation probably were formed authigenically in seasonal lakes and swamps from weathering of volcanic debris, with their different lithologies due to different drainage conditions and parent ash. The Pryor conglomerate member of the Cloverly formation and lenses of similar conglomeratic sandstones in the Little Sheep member were derived from reworked sedimentary rocks west of the depositional area. Channel-filling clean quartz sandstones of the Himes member were derived from an eastern sedimentary, and perhaps metamorphic, terrain. Thinly interbedded sandstones and shales of the Sykes Mountain formation are tidal-flat and other shallow-water beds deposited at the periphery of the transgressing Early Cretaceous sea.

Slow deposition of these well-sorted and mature detrital sediments and close adjustment of authigenic minerals to prevailing environments depended largely on the stable tectonic conditions. During Cloverly time, very little aggradation except of volcanic debris took place, so that soils were formed and preserved.

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