Crevasse Splay Sandstone Geometries in the Middle Jurassic Ravenscar Group of Yorkshire, UK
- M. Marzo and
- C. Puigdefábregas
Published Online: 14 APR 2009
Copyright © 1993 The International Association of Sedimentologists
How to Cite
Mjøs, R., Walderhaug, O. and Prestholm, E. (1993) Crevasse Splay Sandstone Geometries in the Middle Jurassic Ravenscar Group of Yorkshire, UK, in Alluvial Sedimentation (eds M. Marzo and C. Puigdefábregas), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303995.ch13
- Published Online: 14 APR 2009
- Published Print: 16 SEP 1993
Print ISBN: 9780632035458
Online ISBN: 9781444303995
- Crevasse splay sandstone geometries;
- accumulation of Crevasse splay deposits;
- most dominant- ripple and flat lamination;
- crevasse splay sandstones - lobe-shaped;
- coalescing crevasse splay lobes along a trunk channel;
- correlating a crevasse splay sandstone within composite crevasse splay sequence
Two types of crevasse splay sandstones have been defined: (i) small-scale single crevasse splay sandstone lobes attached to the levee of the fluvial channel, and (ii) large-scale composite crevasse splay sandstones associated with crevasse subdelta lobes which infill interdistributary bays.
Sections through single crevasse splay sandstones in the Ravenscar Group show width/thickness ratios less than 1500 and length/thickness ratios less than 2000. Thicknesses are up to 2.5 m and widths and lengths are of similar magnitude (up to about 2000 m). The single crevasse splay sandstones thin rapidly at their outer margins and also outward from the proximal part towards the unconfined crevasse splay lobes. Sections with low width/thickness ratios (10–100), large thicknesses (1.0–2.5 m) and small widths (18–200 m) represent proximal confined crevasse splay sandstones. The geometries of single crevasse splay sandstones in Yorkshire are similar to the geometry of other crevasse splay sandstones described in the literature.
Composite crevasse splay sandstone sequences (usually between 2.5 and 6 m thick) have probably been formed by crevasse subdelta sedimentation. The extent of a composite crevasse splay sandstone level in the Gristhorpe Member is at least 20 km. In the medial to distal part of a crevasse subdelta the composite crevasse splay sequence is composed of single crevasse splay sandstones with widths of around 0.5 km. The proximal part of a crevasse subdelta contains, axially, channelized crevasse splay sandstones fringed by crevasse splay sandstone sheets.
The rock volume of crevasse splay sandstones is strongly dependent on thickness: a doubling of the thickness causes an eightfold increase in volume. The width/thickness ratio also strongly influences the volume estimate: a doubling of this ratio will cause a fourfold increase in volume.
The thickest crevasse splay sandstones are genetically connected to the top part of their fluvial feeder channel. The crevasse splay sandstones are probably in most cases in contact with a fluvial channel sandstone through a crevasse channel sandstone with a low width/thickness ratio (10–20). Sometimes, crevasse splay sandstones are cut by younger fluvial channels.
The crevasse splay sandstones within overbank sequences may place constraints on the location of fluvial channel sandstones; in particular, these constraints are useful when considering sequence architecture in fluvio-deltaic reservoirs.
The quantitative geometrical data increase our ability to make geometric predictions based on well data, and also provide constraints on the dimensions and shapes of sandbodies. Incorporation of this information into a reservoir model will reduce the uncertainty in the model and contribute to more reliable predictions of reservoir performance.