Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 709

as indicators of past sea-level position. Fossil fiddler crab burrows have not
been reported from Pleistocene carbonate rocks, but, if they should be discov-
ered in the future, they also would be good sea-level position markers.
5.1.2 The Psilonichnus Ichnofacies
The beach and coastal dune ichnocoenoses of the Bahamas fall within and con-
form fully to the definition of the Psilonichnus Ichnofacies, as conceived by Frey
and Pemberton (1987, originally termed the Psilonichnus Ichnocoenose) .The
holotype specimen of P. upsilon ( Frey et al., 1984 ) is a burrow likely formed
by the ghost crab, Ocypode quadrata ( Fig. 11 C and D), and preserved within
Holocene carbonate grainstones of backshore beds on San Salvador.
Ichnocoenosis 4 ( Fig. 9 ) is characteristic of carbonate sand beaches of the
Bahamas and elsewhere in the wider Caribbean and beyond, as well as equivalent
Holocene strata in the Bahamas and the Pleistocene of Bermuda ( Curran, 1994,
2007 ). This ichnocoenosis is dominated by the distinctivemodern burrows of ocy-
podid crabs (e.g., ghost crabs) and P. upsilon ( Fig. 11 E). In the Bahamas, the ana-
log relationship between O. quadrata , its modern burrows, and P. upsilon is well
established ( Curran, 1994; Curran andWhite, 1991; Frey et al., 1984 ). The typical
range of vertical oblique, J-, U-, and Y-shapedmorphologies for P. upsilon occurs
in the Holocene beach-backshore beds in the Bahamas. Vannini (1980) and De
(2005) illustrated an even wider range of morphologies for burrows constructed
by species of Ocypode in modern siliciclastic beaches.
Beaches in the Bahamas tend to be relatively steep and narrow and usually
are backed with heavily vegetated dunes. Burrowing activity of O. quadrata is
effectively confined to the beach backshore zone. Unlike its distribution in
coastal siliciclastic settings, where ghost crabs commonly range well back into
the dunes, Bahamian O. quadrata normally does not inhabit dunes, owing to the
thick vegetation cover which impedes its mobility, and possibly also because of
abundant presence of the common land crab, Gecarcinus lateralis , within dunal
areas ( Curran 1994; Curran and White, 1991 ).
P. upsilon is widespread in Holocene upper foreshore and backshore succes-
sions throughout the Bahamas. In contrast, the backshore zone is not well repre-
sented in Pleistocene beach to dune sequences, and P. upsilon has not been
reported from these rocks ( Curran, 2007 ). However, P. upsilon is common in a
carbonate beach to primary dune coastal exposure of the middle Pleistocene Bel-
mont Formation on Bermuda ( Curran, 1994 : Fig. 3.11), where its occurrence has
real value as a sea-level position indicator. This utilitarian aspect of P. upsilon
likely can be applied to other similar tropical carbonate settings elsewhere.
eolianite, near Whale Point, North Eleuthera. Hammer length ¼ 28 cm. (H) Large cluster burrow in
Holocene eolianite, Hanna Bay, San Salvador. Height of burrow complex ¼ 1.4 m. (I) Cross sections
of stellate burrows (shafts with radiating short tunnels) exposed on horizontal surface of Holocene
eolianite, same location as H. Scale bar ¼ 10 cm.