Environmental Engineering Reference
In-Depth Information
The offshore is commonly dominated by fair-weather deposition,
although in strongly storm-dominated regimes, the offshore complex may
consist of shaly zones interbedded with thin, relatively unburrowed tempes-
tites (e.g., Frey, 1990; Howard and Frey, 1984; Pemberton and Frey, 1984;
Pemberton and MacEachern, 1997; Vossler and Pemberton, 1989 ). Fair-
weather deposits are characterized by pervasive burrowing (BI
4-5), man-
ifest by high diversities and uniform distributions of ichnogenera. The suites
lack domination by individual forms and possess ichnogenera characteristic of
equilibrium behaviors, consistent with fully marine, unstressed settings (cf.
Jumars, 1993; Pianka, 1970 ). The offshore is commonly subdivided into lower
and upper subzones (e.g., MacEachern and Bann, 2008; MacEachern and
Pemberton, 1992 ).
ΒΌ
2.1.1 Lower Offshore
The lower offshore, as defined here, comprises dark, typically organic-rich,
silty shale, commonly thoroughly homogenized via biogenic reworking
( Fig. 4A-C ). Fine- to very fine-grained sandstone beds are present, though
in limited amounts. Thin sandstone beds are predominantly intensely biotur-
bated. In thicker beds, remnant undulatory parallel laminae are preserved
( Fig. 4D ). Thicker sandstone beds tend to show sharp, erosional bases,
with moderately to well-developed parallel lamination. Most burrowing is
restricted to the tops of the laminated sandstone beds, interpreted as distal
tempestites. These distal storm beds are deposited well below fair-weather
wave base, such that fair-weather physical processes are unable to modify
them; such beds, therefore, have a high preservation potential ( Dott, 1983,
1988; Wheatcroft, 1990 ).
Fair-weather deposits of the lower offshore commonly display abundant and
uniformly distributed bioturbation, characterized by a high-diversity ichno-
logical suite that corresponds to a distal expression of the Cruziana Ichnofacies.
Trace-fossil suites comprise a mixture of deposit-feeding, and to a lesser degree,
grazing/foraging traces ( Fig. 4 ). Most of these traces show minimal vertical
penetration of the bed. Under such circumstances, distal storm beds may show
little biogenic reworking, except near their tops, even if the tempestite is
relatively thin (e.g., Pemberton and MacEachern, 1997 ). In low-energy settings,
ichnological suites in this zone may be characterized by a proximal (shallow-
water) expression of the Zoophycos Ichnofacies dominated by grazing, forag-
ing, and mining traces ( Fig. 3 ).
Common ichnotaxa in lower offshore deposits include Helminthopsis ,
Phycosiphon , Chondrites , Zoophycos , Schaubcylindrichnus freyi , Planolites ,
Rosselia socialis , Ancorichnus capronus , Ophiomorpha annulata , Cylindrichnus
concentricus , and diminutive Asterosoma and Thalassinoides (e.g., Fig. 4A-C ).
Less common forms include small Cosmorhaphe (e.g., Fig. 4C and D ), Scolicia ,
Teichichnus , and Palaeophycus .
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