Robert G. Young

Dakota Group of Colorado Plateau

Basal Cretaceous deposits of the Colorado Plateau can be subdivided into two formations on the basis of carbonaceous content. The lower non-carbonaceous unit, the Cedar Mountain formation, consists of mudstones and persistent conglomeratic sandstones which were deposited in an inland floodplain environment. The upper carbonaceous unit, the Naturita formation, consists of carbonaceous mudstone, coal, persistent conglomeratic sandstones, and beach sandstones deposited on or adjacent to the shore of the Mancos sea. Naturita deposits can be traced landward into Cedar Mountain deposits, indicating that they are facies of a larger unit, the Dakota group. Advancing Cretaceous seas reached the eastern edge of the Plateau in late Albian time but did not inundate the entire Plateau until late Greenhorn or early Carlile time. The westward advance of the sea was a halting one. Sharp pulses of basinal subsidence, accompanied by uplift in the source area west of the Plateau, resulted in rapid westward transgressions of the sea. Deposition, essentially confined to periods of quiet following the transgressions, caused some regression of the sea but transgressions exceeded the regressions and resulted in a slow westward advance of the sea. Many pulses of subsidence occurred but five major ones, which were accompanied by uplift in the source area, are reflected in the five widespread orogenic sandstones present in these deposits.

Late Cretaceous Cyclic Deposits, Book Cliffs, Eastern Utah

Well defined cyclic deposits are recognized in late Cretaceous deposits of the Rocky Mountain region. Excellent exposures in east-central Utah exhibit the following sequence of units from the base upward: (1) marine shale, (2) littoral marine sandstone, (3) lagoonal deposits, and (4) coal. These units were in large part deposited simultaneously during a period of slow eastward regression of the Mancos sea; hence, a complete sequence is rare. The slow regression was punctuated by numerous sharp pulses of basinal subsidence which produced rapid, westward transgressions of varying magnitudes. The result was numerous repetitions of four nearly simultaneously existing environments whose deposits constitute a fairly standard sequence of units (the basic cyclothem.) The occurrence of petroleum in lagoonal deposits suggests derivation in situ but may be due to better opportunity for preservation in lagoonal areas. Criteria are presented for recognition of cyclic deposits and their individual components, both in outcrop and in the subsurface. Because of rapid intertonguing and variations along the shore line, long-range correlations are usually unreliable; but if one has a full knowledge of the nature of the intertonguing, correlations over short distances can be detailed and reliable.

Mancos B"Interval of Upper Cretaceous Mancos Shale, Douglas Creek Arch, Northwest Colorado: A "Shelf-Sand"Complex: ABSTRACT"

The Mancos B interval of the Mancos Shale, a major gas producer on the Douglas Creek arch is a shelf-sand complex represented by up to 1345 ft (404 m) of thinly interstratified claystone, siltstone, and very fine to fine-grained sandstone deposited approximately 100 mi (161 km) offshore in the Western Interior seaway. In the subsurface, the Mancos B is subdivided into four major units, ranging in thickness from 175 to 480 ft 953 to 146 m). Each unit thins over the archs crest and, in the process, becomes more sand rich. In outcrops along the southern flank of the arch, the lower Mancos B forms a series of coarsening-upward sequences, 25 to 90 ft (8 to 27 m) thick. A typical sequence begins with silty claystone or bioturbated mudstone at the base, followed upward by bioturbated muddy sandstone, and topped with sandy dolomite (as concretions or thin beds). Sedimentary structures include horizontal lamination, wavy lamination, lenticular bedding, flaser bedding, and ripple lamination. Paleocurrent measurements from ripple foresets have a mean azimuth of 111/sup 0/. Burrows characteristic of the Cruziana ichnofacies are common to abundant in all lithofacies.

Differentiating Shelf and Marine Sands from Deltaic and Brackish-Water Deposits Using Modern Techniques: ABSTRACT

The common occurrence of oil and gas in sandstone within stratigraphic sequences composed of interstratified marine and non-marine sediments, formed in environments at or adjacent to a shoreline, is well known to most geologists. Such gross relations are ascertained readily, but the nature and mode of origin of individual sandstone bodies generally have been of little concern. It is probable that these sequences must include sandstone of both marine and non-marine origin. Marine types to be expected include beach, shoal, and shelf sandstone; whereas, non-marine types include deltaic, estuarian, paludal, and lagoonal sandstone. Knowing the mode of origin of sandstone in a petroleum-bearing sequence should be of considerable importance. One type may never contain oil; some andstone types may contain petroleum in predictable, more permeable zones; some types may be long and sinuous and others broad and sheet-like; some may parallel old shorelines whereas others may be at right angles to them. Positive determination of the genesis of a sandstone, either at the outcrop in the subsurface, is difficult, but can be done with some assurance by using multiple criteria--no one of which is entirely diagnostic and only a few of which may be ascertainable for a particular sandstone body. Parameters of value in determining origin include geometry of the sandstone body, sedimentary structures, log characteristics, composition, nature of boundaries, and composition of surrounding or enclosing sediments. End_of_Article - Last_Page 641------------