Page 237 - Sedimentology and Stratigraphy

224 Shallow Sandy Seas

storm-deposited sands. Second, the organisms that . palaeocurrents – flow directions very variable,
occur in shelf deposits are distinctive of shallow mar- reflecting tidal currents, longshore drift, etc.
ine conditions, either as body fossils, specifically . fossils – often diverse and abundant, benthic forms
benthic organisms that are only abundant in shelf are characteristic
environments, or as trace fossils that display diverse . colour – often pale yellow-brown sands or grey
morphologies (11.7). Third, successions of shelf sand- sands and muds
stones and mudstones may also be associated with . facies associations – may be overlain or underlain
limestones deposited during periods of low supply of by coastal, deltaic, estuarine or deeper marine facies.
terrigenous clastic detritus.
Tempestite beds can superficially resemble turbidites
(4.5.2) because they are also normally graded sand- FURTHER READING
stone beds, with sharp bases and interbedded with
mudrocks. Turbidites are more commonly found in DeBatist, M. & Jacobs, P. (Eds) (1996) Geology of Siliciclastic
deep basin environments (Chapter 16), so distinguish- Shelf Seas. Special Publication 117, Geological Society
ing between them and tempestites provides informa- Publishing House, Bath.
¨
tion about the water depth. The presence of HCS–SCS Fleming, B.W. & Bartholoma, A. (Eds) (1995) Tidal Signatures
in tempestites provides evidence of deposition on a in Modern and Ancient Sediments. Special Publication 24,
International Association of Sedimentologists. Blackwell
shelf, and the ichnofacies association will typically
Science, Oxford.
be more diverse than that found in deeper water
Johnson, H.D. & Baldwin, C.T. (1996) Shallow clastic seas. In:
environments (11.7).
Sedimentary Environments: Processes, Facies and Stratigra-
Characteristics of deposits of shallow sandy seas phy (Ed. Reading, H.G.). Blackwell Science, Oxford; 232–280.
. lithology – mainly sand and mud, with some gravel Suter, J.R. (2006) Facies models revisited: clastic shelves. In:
. mineralogy: – mature quartz sands, shelly sands Facies Models Revisited (Eds Walker, R.G. & Posamentier,
H.). Special Publication 84, Society of Economic Paleon-
. texture – generally moderately to well sorted tologists and Mineralogists, Tulsa, OK; 331–397.
. bed geometry – sheets of variable thickness, large Swift, D.J.P., Oertel, G.F., Tillman, R.W. & Thorne, J.A. (Eds)
lenses formed by ridges and bars (1991) Shelf Sand and Sandstone Bodies: Geometry, Facies
. sedimentary structures – cross-bedding, cross- and and Sequence Stratigraphy. Special Publication 14, Interna-
horizontal lamination, hummocky and swaley cross- tional Association of Sedimentologists. Blackwell Science,
stratification Oxford.