Sedimentation on Continental Margins, IV: Lithofacies and Depositional Systems

  1. D. J. P. Swift4,
  2. G. F. Oertel4,
  3. R. W. Tillman5 and
  4. J. A. Thorne6
  1. D. J. P. Swift1,
  2. S. Phillips2 and
  3. J. A. Thorne3

Published Online: 14 APR 2009

DOI: 10.1002/9781444303933.ch4

Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy

Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy

How to Cite

Swift, D. J. P., Phillips, S. and Thorne, J. A. (1992) Sedimentation on Continental Margins, IV: Lithofacies and Depositional Systems, in Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy (eds D. J. P. Swift, G. F. Oertel, R. W. Tillman and J. A. Thorne), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303933.ch4

Editor Information

  1. 4

    Norfolk, Virginia, USA

  2. 5

    Tulsa, Oklahoma, USA

  3. 6

    Plano, Texas, USA

Author Information

  1. 1

    Department of Oceanography, Old Dominion University, Norfolk, VA 23529, USA

  2. 2

    ARCO Alaska Inc., 700 G St., Anchorage, AK 99501, USA

  3. 3

    Research and Technical Services ARCO Oil and Gas Co., 2300 W. Plano Parkway, Plano, TX 23575, USA

Publication History

  1. Published Online: 14 APR 2009
  2. Published Print: 30 JAN 1992

ISBN Information

Print ISBN: 9780632032372

Online ISBN: 9781444303933

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Keywords:

  • sedimentation on continental margins, IV - lithofacies and depositional systems;
  • dispersal systems and depositional systems;
  • textural maturity and dispersal system;
  • depositional system architecture;
  • textural maturity and basin evolution;
  • lithofacies arrays - response to stratification processes and progressive sorting;
  • depositional system theory;
  • source diastem - depositional system geometry;
  • shallow-marine depositional system class;
  • transgressive shelf regimes - storm-dominated regimes and tide-dominated regimes

Summary

Lithologic variation in shallow-marine sediments is best understood in terms of depositional systems (assemblages of process-related lithofacies). Each depositional system is the product of a dispersal system, or assemblage of flow-linked depositional environments. Two useful criteria for classifying the small-scale depositional systems are: (1) the grain size bias of the regional dispersal system; and (2) the type of local dispersal system that created the deposit.

Dispersal systems on tectonically youthful continental margins (rift margins) tend to be regionally biased towards coarse-grained sediment. In most tectonic settings, however, the primary regional controls of grain size are the base level shifts and fluctuations in sediment input that result in transgressive or regressive situations. Transgressive settings have higher bypassing and reworking ratios, and accumulated coarser lithofacies than do regressive settings. Therefore, a primary criterion for classifying depositional systems is whether they occur in transgressive or regressive settings.

Dispersal systems have a common pattern, which consists of an eroding source environment attached to a series of depositional environments whose linking current regime is characterized by a downstream decrease in competence. The most important fluid dynamical parameter in such systems is the reworking ratio, r = a′/å, where a′ is the minimum depth of erosion, and å is the accumulation per event. Both the fluid power and the accumulation rate decrease downstream, but the fluid power gradient is steeper, so that the reworking ratio also decreases. These gradients may reverse in the distal environment, so that the reworking ratio increases once more.

The resulting depositional systems likewise have a common pattern, which can be deduced from the strata-forming principles outlined by Thorne et al. (this volume, pp. 59–87). They consist of a source diastem that overlies, or is overlain by, a lithofacies succession that becomes increasingly finer grained with distance above or below the diastem. The proximal (near-source) portion of the depositional system is characterized by a condensed section as a consequence of the high reworking ratios that prevailed in the depositional environment (amalgamated sand lithofacies), while the distal environment experiences lower reworking ratios and is characterized by an expanded section (Interbedded sand and mud lithofacies; laminated or bioturbated mud lithofacies). However, in some settings, high reworking ratios also may occur in the the far distal environment, when the decrease in fluid power is more than compensated for by the decrease in sediment input. These distal condensed sections are the classical condensed section of stratigraphers.

Depositional systems are characterized by a variety of spatial arrangements. Conjugate systems are stacked together and are separated by a shared source diastem. Serial systems also share a source diastem and are arranged in tandem, with both systems above or below the source diastem. Interfingering systems share a common distal facies. Superimposed systems have been overprinted by several depositional environments. If migration of the source environment has destroyed one of the depositional systems originally in contact with it, the resulting contact is an occluded contact.

Regressive depositional systems occur as delta mouth bars, as prodelta plumes in response to detachment of the coastal boundary flow, or as regressive shoreface-shelf systems. The distal portions of shoreface-shelf systems may develop seaward facing, constructional, terrace-scarp complexes, with clinoform internal structure. Where the shelf widens or deepens so that the along-shelf flow must decelerate, an along-shelf fining deceleration sheet may result. Relative proportions of lithofacies vary from subsystem to subsystem on regressive shelves, but always with the same spatial relationships (i.e. undergo allometric variation).

Transgressive depositional systems consist of the same lithofacies as regressive depositional systems, but the proportions are often very different. Transgressive sands occur as erosion-truncated, discontinuous, barrier and inlet sand systems below a ravinement surface, cut by erosional shoreface retreat. Several back-barrier subsystems may be present, organized around barrier washover, inlet and tidal channel sources. Transgressive back-barrier systems prograde landward, and are capped by their source diastems. Like regressive systems, they consist of both condensed and expanded sections, but their stratification parameters decrease landward, from the barrier to the lagoonal shoreline, not seaward. The shoreface itself is erosional.

Further seaward on the shelf, transgressive facies occur as thin, discontinuous, coarse-grained sand sheets. These sand sheets overlie the source diastem, which in this case is the ravinement surface, or the associated flooding surface that develops seaward of the lowstand shoreline as the transgression proceeds. The proximal condensed lithofacies (amalgamated sand facies) of coarse-grained sheet sands may extend for hundreds of kilometres across and along shelf; expanded lithofacies are displaced towards or over the shelf edge, or along shelf, towards zones of shelf widening and flow deceleration. Coarse-grained sand sheets may be localized as cape-associated and estuarine shoal retreat massifs, which may be dissected by cross-massif tidal or storm flows into comb-like sand ridge arrays. Sand ridge fields also may occur on the submarine shelf plains between massifs, if the sand supply is adequate.