Abstract

Deltaic sands accumulate in fluvial and tidal channels, distributary-mouth bars, beach ridges, barrier islands, and on delta-front platforms. Fortunately a correlation between environment and resulting facies exists which serves to help the petroleum geologist predict the position, trend and geometry of sands and sandstones within depositional basins where deltas contributed significant quantities of sediment. Recent Mississippi, Niger, Guadalupe (Texas), and Colorado (Texas) deltas are described to indicate criteria used in recognizing facies and also for an understanding of the cyclic nature of delta growth. Similar facies are reviewed for comparison from Paleozoic deltas in the central Appalachians and upper Mississippi Valley.

Deltas, the depositional features formed in response to river-mouth sedimentation, are a function of the rate and coarseness of fluvial supply, depth of water and basin topography, strength of marine currents, subsidence, and eustatic change in sea level. These factors determine the size of the deltaic complex, its geomorphic form, number of subdeltas, nature and amount of imbrication of subdeltas, density of distributary pattern, formation of sheet sands, beaches and barrier islands during constructive and/or destructive phase of development, relative rate of sedimentation and animal activity reflected in ratio of laminated to burrow-mottled structures, position and trend and geometry of lithofacies, and amount of constructive phase preserved.

Recent deltas of the Mississippi and Guadalupe are compounded lobate-birdfoot types indicating river influence over relatively weak marine currents. Birdfoot subdeltas exhibit bar fingers of sand and silt whereas the closely spaced distributaries in lobate subdeltas enable mouth bars to coalesce forming thin wedge-shaped sheet sands. The Colorado delta has evolved from a compounded lobate-birdfoot delta within the bay into a cuspate delta in the Gulf of Mexico. Where marine currents are sufficiently strong, the mouth bar supplies sand to longshore currents which feed barrier islands fringing the subaerial delta as in the Niger delta. Sands in the Niger delta also spread over the delta-front platform developing a sand facies paralleling the shore. Barrier islands, beach ridges and sheet sand also occur in the Mississippi delta but, for the most part, only during the destructive phase. Sediments texturally grade finer from delta tip to flank as well as seaward. Vertical cores of prograding deltas show silts and sands coarsening upward over a gradational contact with underlying prodelta silty clays. Transgressive sediments grade finer upward in cores with sands resting on erosional surface.

Fluvial and tidal channels of the subaerial delta are cut-and-fill features with sediments grading finer upward over erosional surfaces. Distributaries of equal discharge are wider and shallower where eroding sand, Point bars, products of lateral accretion, commonly occur in upper deltaic plain and alluvial plain. Plant fibers and wood fragments are common fossils. Finer grained and darker colored natural levee and flood basin sediments, including crevasse deposits and swamps, accumulate between bifurcating distributaries.

The detrital lens records the progradational history of each delta. Gulf coast deltas are a composite of imbricating subdeltas shifting laterally and seaward whereas the Niger appears as a large arcuate delta. The resulting broad lithofacies occur as concentric facies in the Niger and as radial elements in the Mississippi, Colorado, and Guadalupe deltas. The upper part of the lens may be reworked by transgressing marine currents as the delta deteriorates and subsides, flushing out the clays and concentrating the sand into sheet sands, beach ridges, and barrier islands. The sands appear more burrowed and shelly than sands formed during the constructive phase unless supply limited and progradation slow as in early growth of Guadalupe delta. Ancient examples of deltas also indicate the detrital lens bounded by coals, marine limestones and shales. Where subdeltas overlap flanks and toes of earlier subdeltas, the subdelta lenses appear in approximately the same stratigraphic position but separated by different boundary layers. A single transgressing boundary layer can separate a sequence of imbricating deltas following substantial increase in basin subsidence and/or rise in sea level.

Alluvial plain sediments show many common characteristics with the delta plain. Flood basins are filled with crevasse deposits intermittently. Belt sands, composed of many point-bar deposits, trend down the regional slope. Dendroid channel-fill deposits occur along the margin of belt sandstones. Continued progradation may remove the upper part and possibly the entire delta in places where the migrating channel is deeper than the thickness of the delta. The delta also may be partially or totally eroded by lowering sea level and/or raising land level.

Imbrication of sand and sandstone bodies reflects the importance of gradient and compaction in fluvial sedimentation on the alluvial plain and in deltas.

The properties which are most helpful in a stratigraphic analysis of deltaic facies are grain size, vertical textural gradations, fossils, color, coarse-fraction composition, internal structures, uniformity of bedding, geometry and orientation of facies to shoreline or regional slope, erosional or gradational contacts, chemical and marine sediments intertonguing with detrital lenses, and comparison with similar sand or sandstone bodies in the same basin. Heavy mineral, clay mineral, trace element and isotope studies sometimes can supplement the stratigraphic framework previously established by the other diagnostic properties.