ABSTRACT: There Are No Gaps in the Stratigraphic Record

CROSS, TIMOTHY A., Colorado School of Mines, Golden, CO

As Barrell recognized nearly a century ago, oscillations of base level above or below the Earth's surface, and transits of base level across the Earth's surface, leave a high resolution imprint of the passage of time in the stratigraphic record. Complete, continuous time is recorded by rocks plus hiatal surfaces of erosion, bypass or nondeposition. At a particular geographic location, the proportion of total time recorded by rocks versus surfaces, and the positions of rocks and surfaces within a stratigraphic succession, is a function of the history of change in accommodation space and the position (above, below, or at the Earth's surface) and direction of movement (up or down) of base level. Accommodation space is the continuously varying space that limits the maximum volume of sedim nt that may accumulate and be preserved in facies tracts. Base level describes the time/space transfer of sediment mass and the volume of sediment that actually accumulates.

Accommodation space varies geographically and temporally within base-level transit cycles such that the volume of sediment accumulated in similar depositional environments or facies tracts changes through time and space. For example, during times of base-level rise the volume of sediment accumulated in facies tracts along basin margins (e.g., coastal plain, sabhka, shoreface) will be greater than the sediment volume accumulated in identical facies tracts during base-level fall. This volumetric partitioning of sediment into different facies tracts is independent of temporal or spatial scales and accounts for several other attributes of the stratigraphic record that are fundamental to its interpretation.

Volumetric partitioning controls the type and degree of symmetry of stratigraphic cycles of all scales. At particular geographic positions, some cycles or sequences are asymmetric and record sediment accumulation ouly during either base-level fall or rise; others are symmetric and record both rise and fall. Recognition of these differences leads to formal correlation procedures whereby, in different situations, rocks are correlated to rocks, surfaces are correlated to surfaces, or rocks are correlated to surfaces. It also explains the different types of cycles and cyclothems that have been recognized and named. Because volumetric partitioning is scale independent, self similarity of cycle symmetries and numerous facies attributes exists at multiple scales in strata.

Accompanying this volumetric partitioning of sediment are differences in stratal geometries, facies complexity, lithologic diversity, stratification types, and petrophysical attributes of strata preserved within identical facies tracts in different portions of base-level transit cycles. This facies differentiation is a measure of the degree of preservation and amalgamation of geomorphic elements that existed within the depositional environment. The degree of preservation reflects how time is represented in the stratigraphic record, as either surfaces or rock. As the rate at which accommodation space is created increases, proportionally more time will be represented by rock. Sediments are more completely preserved during rising base level when accommodation space is increasing more rap dly; the resulting strata are diverse, heterogeneous, and complex, and the facies composing them show comparatively less cannibalization and amalgamation of the original geomorphic elements that existed during deposition. In essence, there are specific and distinctive stratigraphic signatures in the facies and facies associations that occupy the different parts of base-level transit cycles. The internal and external attributes of stratigraphic elements of individual facies tracts commonly described in "facies models" and "depositional systems" are not constant through base-level transit cycles.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)