Primary Structures and Sedimentary Environments in Dakota Sandstone, Northwestern Colorado: ABSTRACT

Abstract

Depositional environments in the Dakota Sandstone in northwestern Colorado have been identified by comparisons of primary structures present in both the Dakota and in Recent sediments. Identification also is aided by study of lithologic character, organic content, and geometry of sedimentary units. The Dakota ideally consists of a transgressive sequence which, from base to top, shows the following environments: (1) channel and floodplain, (2) swamp-tidal flat-lagoon, (3) beach, and (4) surf-zone and other shallow sub-littoral deposits. In places the beach and shallow-marine sandstone bodies are absent, and shallow-marine black shale of the overlying Mancos Shale lies directly on tidal-flat sandstone in the Dakota. The sediments of most depositional environments contain one or more significant primary structures. Channel deposits contain high-angle cross-beds in irregular, discontinuous units usually 1 ft. thick or more. These cross-beds are formed on point bars by megaripple migration during flood stages. Floodplain End_Page 2037------------------------------ stratification is regular, parallel, and continuous. Tidal flats are characterized by very thick channel cross-beds, thin irregular beds containing organism burrows and trails, and shale-pebble conglomerate. Beach deposits consist of tabular to wedge-shape bundles of thin, regular, parallel to subparallel to laminae. Surf-zone deposits contain low-angle cross-beds in tabular to irregular, relatively continuous sets 1 ft. thick or less. End_of_Article - Last_Page 2038------------