Abstract: Sheet-Flood Deposition and Deposits

William Lee Stokes

Sedimentologists and stratigraphers have proposed criteria for distinguishing deposits of meandering and braided streams. Such categorizations may actually hamper interpretation by setting unnatural restrictions on natural phenomena. For example, existing genetic classifications seem to leave no place for the fluvial deposits originating over long time periods by sporadic but intensive sheet floods. These deposits are referred to here as "sheet-flood deposits."

Sheet-flood deposits have the following characteristics. They (1) are horizontally extensive but relatively thin; (2) are composed of relatively well-worn, durable, gravellike materials; (3) are underlain by well-marked, water-eroded unconformities with only moderate relief; (4) have clasts derived from distant, usually diverse sources; (5) have primary structures indicative of high-energy transport and deposition; (6) have few indigenous fossils except petrified wood; (7) together with the underlying unconformity, must account for long time intervals; and (8) are of nonmarine origin.

Examples are the Shinarump-Moss Back-Gartra-Higham (Middle? and Late Triassic); Buckhorn-lower Burro Canyon-lower Dakota-lower Cloverly-Lakota (Early Cretaceous?); and Ohio Creek-Dear Point (early Tertiary?).

Geographic and climatic conditions governing formation of ancient sheet-flood deposits are essentially the same as those prevailing in the formation of pediments in present-day semiarid situations, but the scale of operation is greater. Although no large permanent streams rising in centralized higher areas were operating, material nevertheless was transported over long distances by sporadic but energetic fluvial action generated by thunderstorms that could cover any part of the area at any time. The ultimate long-term result is to disperse and spread resistant material rather than to concentrate it into channel deposits.

AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah