A firm ground is formed by
a stiff but uncemented carbonate and/or clastic sediment that, at one
time, was close to the sediment–water interface (Droser et al 2002).
Firm grounds in Holocene settings tend to be exposed at the sediment water
interface after the overlying layers of unconsolidated soft sediment have
been eroded. This erosive surface may be one origin of bedding planes.
The firm conditions exhibited by a sediment are usually the result of
dewatering and compaction. Carbonate or clastic silty to muddy sediments
deposited in the absence of bioturbators tend to dewater rapidly. With
the erosion of the overlying softer sediment, this process of compaction
alone will tend to result in the exposure of a cohesive sediment surface.
The character of the sedimentary structures, particularly Glossifungites
burrows that formed close to the sediment–water interface are used
to identify the setting in which firm grounds were formed.
The erosive ravinement associated with transgressive surfaces is developed
with the onset of a sea level rise across a sediment suface over a lowstand
system tract which may expose these firm grounds that become the site
of colonization by the organisms that collectively burrow the surface
to form the Glossifungites ichnofacies. Examples of these firm ground
surfaces include the transgressive surfaces formed just below the maximum
flooding surfaces of parasequence boundaries. It should be recognized
that, as Pemberton and
MacEachern (1995) explain, that the maximum flooding surface is associated
with anoxic conditions and has not been recognized as burrowed. Any burrowing
is likely associated with the underlying transgressive event!
References (See Link to Ichnology Literature to left)
Mary L. Droser, Soren
Jensen, and James G. Gehling, 2002, Trace fossils and substrates of the
terminal Proterozoic–Cambrian transition: Implications for the record
of early bilaterians and sediment mixing;Geologys, October 1, vol. 99 no.
20 p12573
Pemberton, S.G. and MacEachern, J.A. 1995. The sequence stratigraphic significance of trace fossils in examples from the Cretaceous of Alberta. In: Van Wagoner, J.A., and Bertram, G.T. (eds.). Sequence Stratigraphy of Foreland Basin Deposits - Outcrop and Subsurface Examples from the Cretaceous of North America. American Association of Petroleum Geologists Memoir, 64: 429-475.
