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The ichnological expression of marine and marginal marine conglomerates and conglomeratic intervals, Cretaceous Western Interior Seaway, Alberta and northeastern British Columbia

Dept. of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6

Travis W. Hobbs

EnCana Oil & Gas Partnership, P.O. Box 2850, Calgary, AB T2P 2S5

Although uncommon, conglomeratic successions and their associated interbeds do contain trace fossils that attest to their marine or marginal marine origins. Intervals within the Cretaceous of the Western Interior Seaway of Alberta and northeastern British Columbia contain ichnological assemblages consistent with open bay, estuarine incised valley, shoreface, and transgressive ravinement settings.

Conglomerates and conglomeratic sandstones of open bays are largely confined to bay margins, and are markedly heterolithic, commonly trough cross-stratified, and moderately bioturbated with Diplocraterion, Skolithos, Palaeophycus, Conichnus and Ophiomorpha. Interbedded sandstones and mudstones contain Teichichnus, Planolites, Terebellina, Arenicolites, Bergaueria, Rosselia, Asterosoma, Cylindrichnus, Thalassinoides, and fugichnia.

Burrowed incised valley conglomerates and pebbly sandstones mainly record channel fills of tidal inlets or flood-tidal deltas at wave-dominated estuary mouths. Trace fossil suites comprise Teichichnus, Ophiomorpha, Skolithos, Diplocraterion, Arenicolites, Rosselia, Cylindrichnus, and fugichnia. Intercalated sandstone and mudstone beds are moderately burrowed with Planolites, Teichichnus, Terebellina (sensu lato), Ophiomorpha, Palaeophycus, Asterosoma, Chondrites, Conichnus and fugichnia.

Upper shoreface and foreshore conglomerates are well sorted, clast supported, and display good clast segregation. Bioturbation is uncommon, comprising Palaeophycus, Ophiomorpha, Cylindrichnus, Rosselia and fugichnia. Interbedded sandstones and pebbly sandstones may contain Macaronichnus and Palaeophycus. The Cardium Formation locally contains conglomeratic intervals interpreted to reflect the entire shoreface succession. These conglomeratic intervals are unburrowed, though they overlie Glossifungites ichnofacies-demarcated discontinuities, and grade upward into burrowed shelf mudstones.

Conglomeratic transgressive lags and pebbly sandstones mantle wave or tidal scour ravinement surfaces, and commonly infill palimpsest Diplocraterion, Skolithos, Arenicolites, Thalassinoides, and Rhizocorallium of the Glossifungites ichnofacies, which demarcate these discontinuities. Transgressive lags may be variably burrowed with virtually any softground suite, depending on the depositional setting. Tidal scour ravinement lags at incised valley mouths, for example, may be weakly burrowed with an impoverished Skolithos ichnofacies, whereas wave ravinement lags produced during regional transgression can contain open marine, diverse mixed Skolithos-Cruziana ichnofacies assemblages.