For instance, geostrophic flow may be more common during trans-gressive to early highstand conditions, when accom-modation space is higher and there is greater potential for Coriolis def
Trang 1have enhanced apparent cross-shelf transport
Sedi-mentological data must therefore be combined with
sequence-stratigraphical data to understand better
the bed geometries and transport distances of ancient
tempestites The stratigraphical distribution of
tem-pestites may also be understood within a
sequence-stratigraphical framework, particularly as sea-level
has a strong control on shelf profile For instance,
geostrophic flow may be more common during
trans-gressive to early highstand conditions, when
accom-modation space is higher and there is greater potential
for Coriolis deflection, whereas more offshore-directed
flow may take place during late highstand conditions
characterized by greater bottom friction Tempestites
are probably more abundant during late highstand
to early lowstand systems tracts of third- or
higher-order sequences, when sand is mobilized from the
coast during relative sea-level fall
Carbonate Systems
Although the previous discussion applies to both
car-bonate and siliciclastic systems, there are also
depos-itional features that are specific to, or most common
in, carbonate systems The most abundant such
fea-tures include flat-pebble conglomerate beds and
shell-bed accumulations Flat-pebble shell-beds are an important
component of carbonate deposits, particularly in
Cambrian and Lower Ordovician strata, and are
gen-erally considered to be accumulations of intraclasts
produced by the storm reworking of early-cemented
carbonate in shoreline and shallow subtidal environ-ments Detailed sedimentological analyses indicate that some flat-pebble beds were deposited from storm-generated combined flows, although it is likely that there are many other processes that could produce such beds
Shell-rich grainstone tempestites accumulate as a result of winnowing by storm waves, commonly in shoreline and nearshore settings, and they have char-acteristics that are different from those of tempestites produced in condensed sections within maximum flooding intervals In shallow epicratonic seaways, winnowing and concentration of shell debris occurs during lowstand conditions, and further reworking and basinward transport occurs during early trans-gression Pronounced trends in the thickness, number, and character of shell beds occurred over the Phanerozoic, and these relate to evolutionary trends and complex taphonomic variables
See Also Sedimentary Environments: Depositional Systems and Facies; Carbonate Shorelines and Shelves.Sedimentary Processes: Depositional Sedimentary Structures; Par-ticle-Driven Subaqueous Gravity Processes Sediment-ary Rocks: Mineralogy and Classification; Sandstones, Diagenesis and Porosity Evolution.Unidirectional Aque-ous Flow
Further Reading Arnott RW and Southard JB (1990) Exploratory flow duct experiments on combined flow bed configurations, and some implications for interpreting storm event stratification Journal of Sedimentary Petrology 60:
211 219
Dott RH Jr and Bourgeois J (1982) Hummocky stratifica tion: significance of its variable bedding sequences Geological Society of America Bulletin 93: 663 680 Duke WL (1990) Geostrophic circulation or shallow marine turbidity currents? The dilemma of paleoflow patterns in storm influenced prograding shoreline systems Journal of Sedimentary Petrology 60: 870 883 Einsele G (1996) Event deposits: the role of sediment supply and relative sea level changes overview Sedimentary Geology 104: 11 37
Einsele G and Seilacher A (1982) Cyclic and Event Stratification Berlin: Springer Verlag
Kidwell SM (1991) The stratigraphy of shell con centrations In: Allison PA and Briggs EG (eds.) Taph onomy: Releasing the Data Locked in the Fossil Record,
pp 211 290 Topics in Geobiology, 9 New York: Plenum Press
Kreisa RD (1981) Storm generated sedimentary structures
in subtidal marine facies with examples from Middle and Upper Ordovician of southwestern Virginia Journal of Sedimentary Petrology 51: 823 848
Figure 8 Generalized wave modified turbidite bed Temporal
history of generalized bed is shown in the triangular diagram of
storm related effects (see also Myrow PM and Southard JB (1996)
Tempestite deposition Journal of Sedimentary Research 66: 875
887) HCS, hummocky cross stratification.
586 SEDIMENTARY ENVIRONMENTS/Storms and Storm Deposits