Environmental Engineering Reference
In-Depth Information
4.1.1 Valley Margins and Substrate-Controlled Suites
During relative sea-level fall, valleys are incised into underlying successions
forming an SU and served as zones of sediment bypass. During late lowstand
and early transgression, however, aggradation and eventually retrogradation
dominate the valley-fill architectures. In the Crystal Field, the valley floors
and margins are demarcated by firmground omission suites, indicating that
the initial preserved facies of the valley were marine or marginal-marine in ori-
gin, indicating a transgressive surface developed on the SU (TS/SU). The TS is
non-appreciably erosional within the bay and along the bay margins, but ranges
from a TRS near the mouth of the system, where it is associated with the tidal
inlets, to a wave-ravinement surface (WRS) once the valley is filled and trans-
gression has breached the barrier fronting the system.
Infaunal colonization of the TS/SU may yield suites attributable to all three
substrate-controlled ichnofacies, depending upon the character of the substrate
that was exhumed. In the Crystal Viking IVF, however, only suites of the Glos-
sifungites Ichnofacies are preserved along the valley margins and consist of
firmground Diplocraterion , Thalassinoides , Rhizocorallium , Gastrochaeno-
lites , and Skolithos . The valleys are excavated into coarsening-upward, region-
ally extensive, fully marine parasequences of the underlying highstand systems
tract ( Pattison and Walker, 1994 ). These stacked parasequences consist of thor-
oughly bioturbated silty mudstones, sandy mudstones, and muddy sandstones,
containing fully marine, high-diversity ichnological suites attributable to distal-
to-proximal expressions of the Cruziana Ichnofacies.
4.1.2 Trace-Fossil Distributions by Subenvironment
The bay-head delta complex (i.e., inner estuary) is characterized by sediments
supplied by the river and deposited into the central basin as lobate, typically
river-dominated deltaic wedges ( Fig. 10 ). Facies are largely characterized by
interstratified planar-laminated and current-rippled sandstones, as well as mod-
erately burrowed sandstones and muddy sandstones ( Pattison and Walker,
1994 ). The sandstones are associated with the delta front and the distributary
channels. Mudstone interlaminae and interbeds are common. Bioturbation
intensities are generally low (BI
0-2), and burrowed horizons are sporadically
distributed. The ichnological suites are dominated by Planolites , Palaeophycus ,
Skolithos , Ophiomorpha , and fugichnia, with subordinate Teichichnus , Areni-
colites , Cylindrichnus , Rosselia , and Diplocraterion ( Fig. 10 ). Rare suites
may contain isolated occurrences of Conichnus , Bergaueria , Asterosoma ,
Schaubcylindrichnus freyi , Lockeia , Macaronichnus , Thalassinoides , and
Siphonichnus ( MacEachern and Gingras, 2007 ).
The central-basin complex ( Fig. 10 B) represents the standing body of water
lying behind the barrier (middle estuary) and ranges from successions that are
sand dominated (bay margin) to those that are mud dominated (deeper bay).
Whether sand or mud dominated, the central-basin deposits consist of two
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