SEPM Strata

tempestites distinguished from turbidites

Sediment deposition and reworking associated with storms (tempestites) and turbidite currents (turbidites) are unpredictable, sudden, and catastrophic. Both experience many of the same processes, have similar character and so are difficult to distinguish from each other.
tempestites are the products of storms that produce waves and currents that extend to and just below wave base in shallow shelf settings. In contrast turbidites are the products of the suddent mobilization of sediment on the shelf margin by the sediment surface over-steepening on the sea floor and sudden movement downslope triggered potentially by too much sediment, storms and/or earthquakes.
Both tempestites and turbidites are the products of events that are "aperiodic" and of varying magnitudes, the larger the more rare. The larger events wipe out the signatures of earlier smaller ones. These dicyclic or non-periodic sequences are caused by irregular stratigraphic events, often the recurrent processes associated with the depositional regime (autocyclic processes) (Einsele et al , 1991). The sedimentary cycles of both tempestites and turbidites tend to be composed of graded beds that fine upward, form symmetric or asymmetric cyclic bundles of sequences, as well as complete and incomplete cycles, the latter reflecting non-deposition and/or erosion during the depositional cycle. Recurring processes generate successions of bundles cyclic beds. Myrow in a series of publications, sometimes alone and sometimes with others (see below), has developed models that explain the heirachy of expected sedimentary structures associated with tempestites and also considers how the surfaces of shallow turbidites are modified by waves.

The table and attached diagram below suggest strategies for separating tempestite deposits from those of turbidites.

		tempestites	turbidites
	Wave ripples and wave ripple cross lamination	Common (apart from distal types)	Absent
	Current ripples and current ripple bedding	Less common than in turbidites	Common
sedimentary structures	Convolute lamination	Rare	Common
(from top to bottom of bed)	Hummocky cross-stratification	Common	Absent
	Traction carpet with inverse grading	Absent	Common in proximal types
	Nature of sole marks	Often bipolar, pronounced irregular scouring, gutter-casts channeling	Uni-directional
	Benthic background community (in muddy intercalations)	Shallow water fauna, differing with substrate consistency	Deep-water fauna, mainly represented by burrows
Biofacies	Displaced body fossils within event beds	Shallow water species only	Shallow and deep-water
	Autochthonous post-event fauna and bioturbation	Fauna similar to pre-event fauna (return to background fauna, if substrate similar)	Episodic colonization by specific fauna preceding return to background conditions
	Amalgamation	Very common and pronounced including "maturation" of sediment	Less common, no maturation effect
Stratigraphic context	Continuity of single beds	Mostly limited	Often over wide distance
	Thickness of sequence	Limited associated with shallow-water facies	In general great, associated with deep-water facies

Table (After Einsele et al, 1991)

References
Einsele G., Ricken W., and Seilacher A., (editors), 1991, "cycles and events in stratigraphy", Springer-Verlag, Berlin, Heidelberg, New York 1991. 955p.
Myrow, P.M. and Southard, J.B., 1991, Combined-flow model for vertical stratification sequences in shallow marine storm-dominated beds: Journal of Sedimentary Petrology, v. 61, p. 202-210.
Myrow, P.M., 1992, Bypass-zone tempestite facies model and proximality trends for an ancient muddy shoreline and shelf: Journal of Sedimentary Petrology, v. 62, p. 99-115.
Myrow, P.M., 1992, Pot and gutter casts from the Chapel Island formation, southeast Newfoundland: Journal of Sedimentary Petrology, v. 62, no. 4, p. 992-1007.
Myrow, P.M., 1994, Pot and gutter casts from the Chapel Island formation, southeast Newfoundland — reply: Journal of Sedimentary Petrology, v. A64, p. 706-709.
Myrow, P.M. and Southard, J.B., 1996, tempestite deposition: Journal of Sedimentary Research, v. 66, p. 875-887.
Myrow, P.M., Fischer, W., and Goodge, J.W., 2002, Wave-modified turbidites: combined-flow shoreline and shelf deposits, Cambrian, Central Transantarctic Mountains: Journal of Sedimentary Research, v. 72, p. 641-656.