Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 563

Upper shoreface deposits are distinctive in the dominance of multidirec-
tional trough cross-stratification, typically in 15-45 cm thick sets, interbedded
with low-angle bidirectional planar cross-bedded sets ( Davidson-Arnott and
Greenwood, 1976; Reinson, 1984; Roy et al., 1980; Thom et al., 1986 ). Sandstone
units are typically well sorted, well winnowed, medium grained, or coarser. Con-
glomerate and pebbly sandstone intervals tend to have a moremassive appearance
and contain low-angle planar-stratified beds. Heavy-mineral concentrations
may exceed those into the foreshore zone ( Howard and Frey, 1984 ). Wave-
induced liquefaction may locally produce convolute bedding and dewatering
structures.
Trace fossils are locally common but rarely abundant, and diversities are
characteristically low. Continuously migrating bedforms present a major eco-
logical problem for endobenthic organisms; consequently, there is a general
scarcity of animals that are capable of constructing permanent domiciles
under such conditions ( Howard, 1972, 1975; Saunders, 1989 ). Those biogenic
structures that are formed have a low preservation potential and, in general,
only deeply penetrating structures are preserved ( Howard and Frey, 1984 ).
The main ichnological elements correspond to the Skolithos Ichnofacies
( Fig. 8 ) and are characterized by the ichnogenera Skolithos ; heavily lined, ver-
tical, or deeply penetrating Ophiomorpha nodosa and Conichnus conicus
( Fig. 8B ). Under very high-energy conditions, Macaronichnus isp. and
M . segregatis are also locally abundant ( Fig. 8C ), such as in the Bluesky
Formation ( Moslow and Pemberton, 1988 ), Cadotte Member ( MacEachern
and Pemberton, 1992 ), the Appaloosa Sandstone of the Bearpaw-Horseshoe
Canyon transition ( Saunders, 1989; Saunders and Pemberton, 1986 ), and
the Blood Reserve Sandstone ( Nadon, 1988 ) of the Western Canada Sedimen-
tary Basin. Macaronichnus is typically more common near the upper shore-
face/foreshore contact ( Pemberton et al., 2001; Saunders, 1989; Saunders and
Pemberton, 1986; Saunders et al., 1994 ). Under more reflective shoreface
states, characterized by steeper depositional profiles and a larger grain size,
the zone may occur lower down in the upper shoreface. The Cadotte Member
contains upper shoreface intervals consisting of conglomerates interbedded
with medium- to coarse-grained sandstone beds, many of which possess
intensely burrowed zones of M. segregatis , demonstrating its high-energy
affinity ( Saunders et al., 1994 ).
Unlike the lower and middle shoreface, storm effects in the upper shoreface
are generally reflected by ridge and runnel systems rather than major deposi-
tional events. Ridge and runnel systems indicate erosion of the beachface
and transport of sediments to the shoreface, related to short-period waves and
wave setup. Poststorm recovery is manifest by landward migration of ridges
and longshore bars, which weld to the beachface in response to long-period
fair-weather waves ( Davies et al., 1971 ).