Ichnology of the Pliocene Casa Cruz Member (Moruga Formation), Trinidad, West Indies: Delineating depositional environments and environmental stresses

Abstract

Integrated sedimentologic and ichnologic analysis of outcrops of the Casa Cruz Member (Moruga Formation, early to mid-Pliocene) yields distinct facies associations representing river- and wave-dominated deltaic deposits, and non-deltaic shoreface successions. This study demonstrates how river and wave processes interacted and their products were preserved across a small geographic area. It also shows evidence of the impact of these processes on infauna and epifauna. Wave-dominated delta front deposits comprise hummocky and swaley cross-stratified tempestites and wave-rippled sandstone beds. Event sedimentation and heightened water turbidity associated with frequent or intense storm events generated low intensity, sporadically distributed, reduced diversity trace fossil suites of the Rosselia ichnofacies. Wave-dominated prodelta deposits display diminutive structures ascribable to the Phycosiphon ichnofacies. Wave-dominated deposits transition into non-deltaic shoreface successions that contain densely bioturbated sediment with high diversity trace fossil suites belonging to the Cruziana ichnofacies. Common traces are Scolicia, Macaronichnus, Ophiomorpha, Thalassinoides, Planolites, Palaeophycus, and Skolithos. In contrast, river-dominated delta front and prodelta deposits are characterised by sandstone, siltstone and mudstone beds devoid of bioturbation. Syn-sedimentary deformational, current-generated structures, and organic particulates are abundant. Hyperpycnal discharge and concomitant increased sedimentation rates, freshets, and phytodetrital pulses linked to seasonal floods are possibly due to the presence of the Intertropical Convergence Zone at that time. This discharge generated extreme physical and chemical stress that inhibited benthic colonisation.