RECOGNITION OF BRACKISH-WATER TRACE-FOSSIL SUITES IN THE CRETACEOUS WESTERN INTERIOR SEAWAY OF ALBERTA, CANADA
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| Authors: | MacEachern, JA; Gingras, MK |
| Year: | 2007 |
| Journal: | 88: 149+ |
| Title: | RECOGNITION OF BRACKISH-WATER TRACE-FOSSIL SUITES IN THE CRETACEOUS WESTERN INTERIOR SEAWAY OF ALBERTA, CANADA |
| Abstract: | The concept of "brackish water" encompasses a wide range of salinities that occupy a continuum between fresh water (< 2 parts per thousand salinity) and fully marine waters (35 parts per thousand salinity). Ichnological assemblages in brackish-water settings are inherently variable, as a consequence of temporal and geographic variations in such factors as average salinity, magnitude of salinity fluctuations, nature of the colonized substrate, presence and/or duration of subaerial exposure, and fluctuations in hydrodynamic energy. Trace-fossil suites attributable to brackish-water conditions are recognizable as departures from archetypal, fully marine expressions of recurring ichnofacies. These departures are characterized principally by impoverishment of assemblage diversities. Facies-crossing forms, Such as Planolites, Teichichnus, "Terebellina", Cylindrichnus, Skolithos, Palaeophycus, and Ophiomorpha, are persistent, particularly with decreasing salinities and/or increasing environmental stresses. Ichnogenera that are largely "confined" to the open-marine realm versus those "diagnostic" of brackish-water environments are variable, and depend in part upon the nature of the adjoining open system and the age of the study interval. Nevertheless, with increasingly stenohaline conditions, ichnogenera such as Helminthopsis, Helminthorhaphe, Cosmorhaphe, Phycosiphon, Zoophycos, Chondrites, Asterosoma, Rosselia, Rhizocorallium, Bergaueria, Schaubcylindrichnus, Scolicia, and Conichnus tend to be more recurrent. Additional aspects of brackish-water suites include size reduction of ichnogenera, introduction of monogeneric and monospecific suites, and repeated juxtaposition of facies-crossing elements of both the Skolithos Ichnofacies and the Cruziana Ichnofacies. This paper summarizes the ichnological characteristics of a large number of well-studied Cretaceous intervals that reflect brackish-water deposition. These units define four broad settings: (1) restricted or barrier-barred bays (e.g., Ostracode Zone, Paddy Member, and the Grand Rapids Fm); (2) open, non-barred bays (e.g., McMurray Fm, Basal Colorado Sandstone, Viking Fm, and Ben Nevis Fm); (3) riverine estuaries (e.g., McMurray Fm, Grand Rapids Fm, Waseca Fm, Glauconite Fm, and Ostracode Zone); and (4) barred, wave-dominated estuaries (e.g., Viking Fm, Bluesky Fm, and Bow Island Fm). Subenvironments within these broad groupings can be further delineated, using a combination of ichnological characteristics and primary sedimentological features. Although general ichnological trends from fully marine conditions into brackish-water settings can be determined, the aforementioned complexities serve to inhibit the development of a universally applicable, recurring brackish-water "ichnofacies". As such, brackish-water deposits are best characterized by comparison with their genetically related open-marine counterparts. |
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