Sedimentation on Continental Margins, III: The Depositional Fabric—an Analytical Approach to Stratification and Facies Identification
- D. J. P. Swift5,
- G. F. Oertel5,
- R. W. Tillman6 and
- J. A. Thorne7
Published Online: 14 APR 2009
Copyright © 1991 The International Association of Sedimentologists
Shelf Sand and Sandstone Bodies: Geometry, Facies and Sequence Stratigraphy
How to Cite
Thorne, J. A., Grace, E., Swift, D. J. P. and Niedoroda, A. (1992) Sedimentation on Continental Margins, III: The Depositional Fabric—an Analytical Approach to Stratification and Facies Identification, 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.ch3
Norfolk, Virginia, USA
Tulsa, Oklahoma, USA
Plano, Texas, USA
- Published Online: 14 APR 2009
- Published Print: 30 JAN 1992
Print ISBN: 9780632032372
Online ISBN: 9781444303933
- sedimentation on continental margins, III - depositional fabric—an analytical approach to stratification and facies identification;
- episodic and stochastic nature of shallow marine flow creating stratified deposits;
- storm bed stratification analytical theory - theory of storm bed generation and concept of dynamic bedload armouring;
- storm bed generation;
- sediment transport formulations;
- power law approximation to bed thickness;
- parametric approach to bed preservation;
- Mesaverde Formation character;
- quantitative model of bedthickness cross-section
This paper presents an analytical approach to the problem of storm bed formation and preservation in the shallow marine environment. A random series of storms are assumed to rework and aggrade the depositional surface. Each storm reworks a thickness, Δη, and deposits a small amount of additional sediment such that the accumulation rate remains a constant, å. The storm bed thickness, Δη, is calculated as a function of the size class of each storm. In general, larger storms, which rework a greater amount of sediment Δη, occur less frequently than smaller storms. Storms can therefore be classed by their return period, T. An earlier formulation for computing storm bed thickness has been modified to take into account the role of bedload armouring as a control on storm bed thickness. In the formulation, a sediment transport computation is used to predict: (1) the mean, preserved, event bed thickness: and (2) the percentage of the sedimentary column consisting of identifiable single-event beds. Parameters for the sediment transport computation include water depth, grain-size distribution, accumulation rate and wave climate parameters α and β, which determine the return period of storms of various sizes.
The results of the computations are used to develop an analytical model for storm bed thickness and preservation. The model is built around two basic relationships. In the power law approximation, Δη = a′TP, where Δη is bed thickness for a storm of a given return period T, p is return period power and a′ is the return period intercept, or the minimum bed thickness. A second basic relationship describes the extent of preservation of the deposited bed in terms of a reworking ratio, r, the ratio of the minimum bed thickness a′ to the accumulation rate, å. As r increases, more of each deposited bed is resuspended by the next event and incorporated into the next bed. Knowing, r, it is possible to estimate the cut-off percentage, c%, which is percentage of the bed that must remain for successive deposits to be identifiable as single-event beds.
The results of the model are applied to measured sections of bed thickness in the Upper Cretaceous Mesaverde Formation of northwestern Wyoming. Model parameters are estimated from the palaeogeography of the Western Interior Seaway. The computations show that water depth and accumulation rate vary characteristically with stratigraphic sequence position. The preferred interpretation shows that fifth-order cycles within the Mesaverde Formation are created by sea-level cycles of approximately 100 000 years duration and 2 m amplitude.
More generally, our study shows that the reworking ratio, r, is an important indicator of the stratification pattern in shallow marine settings. It shows that shifts in regime scale parameters (accumulation rate; å, sea-level change rate, R; sediment input rate, Q) are expressed at subequilibrium (event) scales by changes in the event parameters a′, r and c%. These changes in turn modify the stratification and grain-size patterns, which taken together constitute sedimentary facies.