Ichnological and Sedimentological Criteria for Differentiating Brackish-Water Bay-Head Deltas and Fully Marine Open-Coast Deltas

Cameron R. Thompson, James A. MacEachern, and Aaron J. DesRoches
Earth Sciences, Simon Fraser University, Burnaby, BC, Canada

Differentiating embayment-bound bay-head delta successions and open-coast delta complexes is essential for accurate paleoenvironmental interpretations and paleogeographic reconstructions. Bay-head delta successions of the Lower Cretaceous Grand Rapids Formation are characterized by coarsening-upward heterolithic intervals with abundant fluid mud drapes, rare syneresis cracks, thin (<10cm) sandstones with micro-HCS, oscillation and current ripples, and normally graded, locally laminated-to-burrowed sandstone to siltstone beds. Bioturbation intensities range from BI 1-6. Trace fossil suites of bay-head delta successions are broadly consistent with existing brackish-water ichnological models: suites are of low diversity, contain diminutive ichnogenera, form depauperate marine ichnocoenoses, and are dominated by facies-crossing forms. Common ichnogenera include Gyrolithes, Cylindrichnus, Lingulichnus, Skolithos, Planolites, rare Chondrites, Lockeia, navichnia, and fugichnia.

Deltaic systems interpreted to have prograded into fully marine basins also comprise coarsening-upward successions with claystone drapes of fluid mud origin. Units show more abundant syneresis cracks and oscillation ripples, thicker HCS, and rare current ripples, trough cross-beds normally graded sandstone to siltstone beds, and carbonaceous laminae. Ichnogenera are more robust than brackish-water counterparts, and display increased diversity. Bioturbation intensities range from BI 0-5. Common ichnogenera include Cylindrichnus, Skolithos, Planolites, Thalassinoides, Chondrites, Lockeia, Teichichnus, Asterosoma, Rosselia, Taenidium, Phycosiphon, rare Zoophycos, Gyrolithes, Phycosiphon, Rhizocorallium, navichnia, and fugichnia. Notable is the sporadic presence of elements that are interpreted to record the activity of organisms deemed intolerant of physico-chemical stress.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas