Lithostratigraphy versus chronostratigraphy in facies correlations of Quaternary deltas: Application of bedding correlation

Outcrop and high-resolution seismic studies show that prograding delta deposits consist of seaward-dipping, offlapping clinoform strata. Despite this, many studies of Quaternary deltas, particularly those based on correlation of sediment cores, commonly depict sharp to gently undulating facies boundaries, similar to those originally shown by Scruton in 1960. The Scruton model emphasizes "layer-cake" lithostratigraphy that correlates similar-appearing but highly diachronous environmental facies, bounded by solid lines that cut across time lines.

In contrast, facies architectural and sequence stratigraphic studies of ancient subsurface deltas have largely abandoned this lithostrati-graphic approach. The alternate "chronostratigraphic" approach uses outcrop and seismic examples as training images that are used to derive conceptual models that drive the correlation of the internal facies architecture of subsurface strata. These outcrop and seismic examples suggest that there is no observable physical boundary between Scruton's diachronous facies units. The conceptual "norm" depicts prograding deltas as seaward-dipping clinoform strata. Dipping delta-front sandstone beds roughly parallel time lines and interfinger with muddy prodelta bottomsets. If individual beds cannot be resolved, then diachronous, transitional facies boundaries are routinely drawn in a way that indicates that boundaries of this type are gradational rather than sharp, specifically by using lightning-stroke-type "shazam" lines.

We use the method of bedding correlation (i.e., correlation of beds and bedsets) derived from geometries observed in outcrops and seismic analogs as a conceptual guide to recorrelate beds and facies for several recently published modern examples, where data are limited to a few, widely spaced cores. The new correlations, although imprecise because of long correlation distances, are potentially more accurate depictions of the bed-scale facies architecture, and may be more useful in applications that involve modeling bed-scale growth of deltas or that require prediction of 3-D fluid-flow behavior of deltaic reservoirs and aquifers.