--> ABSTRACT: Utility of Allostratigraphic Units (Formal) and Their Relationship to Sequences (Informal), by James B. Stevens; #91002 (1990).

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ABSTRACT: Utility of Allostratigraphic Units (Formal) and Their Relationship to Sequences (Informal)

James B. Stevens

Allostratigraphic units are boundary-defined material units established by the North American Stratigraphic Code. Explicitly, the boundaries are discontinuities, exemplified by unconformities, and pedostratigraphic units, both of which involve changes in conditions, and hiatus recognized to be significant for the purpose of a stratigrapher. Marked discontinuity in style of deposition (marine versus nonmarine, for example) also involves significant change, with or without hiatus, and should meet the spirit of the definition of allostratigraphic units. Use of allostratigraphic units has advantages that include the lack of weighty connotations carried by the term sequence, and the emphasis on change given by the need to define boundaries at discontinuities. Furthermore, reco nition of sequences landward beyond the limits of paralic deposition goes beyond the bounds of the concept, but allostratigraphic units are definable for most depositional circumstances. Allostratigraphy emphasizes depositional, nondepositional, and/or erosional events, while lithostratigraphy emphasizes lateral continuity of the effect of an event. Sequences are a special kind of allostratigraphic unit, for which content and boundaries are generated by transgression and regression, which are in turn driven by a set of causes, those capable of producing eustatic sea-level change. For example, cyclothems could be considered allostratigraphic units, particularly if the definition were slightly extended as suggested. But a cyclothem in, or supplied with sediment by, tectonically active sett ngs might not correspond exactly to sequences in less active areas. The uncertainty of the connection with eustatic causes might also vary directly with distance from the nearest continental margin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990