Subaqueous deltas in the stratigraphic record: Catching up with the marine geologists

Abstract

Delta bathymetry, seismic data and near-surface sediment sampling on modern deltas with significant wave, tidal or marine current influence betray a double clinoform architecture with a bridging subaqueous platform. Much of the muddy portion of river-sediment discharge that reaches the coastline bypasses the mouth bar/shoreline clinoform and is deposited, eroded, re-suspended and stored in the distant subaqueous portion of deltas. The sediment stored on the prograding slope of the subaqueous delta is predominantly muddy and heterolithic sediment gravity flows, a diagnostic feature of the prograding and rapidly accumulating subaqueous delta. The subaqueous delta sometimes becomes markedly skewed offshore to run parallel to the shoreline, a routing often aided by shelf currents. Early marine researchers tackled the problem of how sediment from the river reaches so far out (commonly 100 km) on the shelf; they showed that negatively and positively buoyant river plumes, and reworked delta front/shoreface sediments are dispersed out onto the subaqueous delta, greatly assisted by the action of waves, tides and especially friction-reducing fluid mud on the seabed of the subaqueous platform. Documentation of the growth of modern subaqueous deltas has contributed to recent progress in understanding mud dispersal on shelves. Equivalent understanding of ancient deltas, however, has lagged behind.

A limited dataset of ancient, double-clinoform deltas has nevertheless strengthened our understanding of how lithology and facies change across the subaqueous deltas. The ancient examples, particularly in well-resolved seismic data as on the Indus Delta and New Jersey shelf, show that the subaqueous delta clinoforms can be distinguished clearly from the mouth bar/delta front or shoreline clinoforms. However, architectural reconstruction from outcrop or well-log data is less simple. The diagnostic two-tier architecture of ancient double-clinoform successions (often eye-catching where the upper sandy shoreline deposits sit abruptly atop the underlying muddy subaqueous delta deposits) is frequently delineated by a continuous or discontinuous erosion surface that vertically separates the two tiers. This is the subaqueous platform surface of sediment bypass onto the subaqueous delta clinoform. Most bypassed sediment accumulates on the gentle foreset and flattening bottomset of the subaqueous delta to produce a 10s of m-thick, upward-coarsening muddy to heterolithic succession with tell-tale thin interbeds of rippled, graded and wavy-laminated tempestite and gravity-flow ‘event’ beds. The subaqueous platform is composed of variably thick mudstone and sandstone beds. The upper tier (ca. 5-15 m-thick) above the subaqueous platform is commonly sand-prone, but may also be muddy, and represents the delta-plain to shoreline clinoform. Additional features that help identify the compound delta are (1) rapid termination of the shoreline deposits, then fronted only by the subaqueous delta, (2) very low angle of downlap of the muddy subaqueous delta strata onto the shelf, and (3) accumulation of minimally bioturbated fluid-mud beds on the inner subaqueous platform, often causing the subaqueous delta succession to have a fine-grained capping, below the erosion surface. A key aspect of the present work is to show how unbioturbated fluid mud units, thin sand beds, muddy sediment gravity flows and bioturbated muds are variably distributed across the subaqueous delta.