Sedimentological and ichnological signatures of changes in wave, river and tidal influence along a Neogene tropical deltaic shoreline

Abstract

Analysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.

Variability In Sedimentological and Ichnological Signatures Across A River-Dominated Delta Deposit: Peay Sandstone Member (Cenomanian) of the Northern Bighorn Basin, Wyoming, U.S.A

“…Additionally, Hansen and MacEachern (2007) were among the first to consider ichnological variability along depositional strike in a single lobe of an ancient asymmetric, mixed river-waveinfluenced delta system. Similar relationships have also recently been inferred in drillcore data from Neogene deltaic successions in Venezuela by Buatois et al (2012). Such palaeogeographic variability in trace-fossil assemblages, however, has not yet been mapped within a continuously exposed, single deltaic sandstone body.…”

“…Most studies recognize that deltas are dynamic depositional systems and are best described as mixed-energy environments that may change in their balance of physical processes and thus in their resulting deposits both in space and in time (e.g., Eagar et al 1985;Ekdale and Lewis 1991;MacEachern et al 2005;Hansen and MacEachern 2007;Sadeque et al 2007;Gani et al 2007;Bann et al 2008;Carmona et al 2009;Dafoe and Pemberton 2010;Li et al 2011;Buatois et al 2012). Hansen and MacEachern (2007) and Dafoe and Pemberton (2010) studied the basal Belly River Formation and Viking Formation, respectively, in central Alberta, focusing on the sedimentological and ichnological variations between river-influenced and waveinfluenced deltaic deposits.…”

“…This is not merely a semantic problem, because a focus of recent research has been the identification of ''ichnological signatures'' of particular deltaic styles (principally river-vs. wave-vs. tide-dominated end members; MacEachern et al 2005;Gani et al 2007;Coates and MacEachern 2007;Li et al 2011). Such signatures can be used as diagnostic tools in the analysis of subsurface datasets such as drillcore (e.g., Buatois et al 2012), and predictions of sediment-body geometry and flow properties based upon ichnological premises. If individual deltas display inherent lateral variability in their ichnological signatures, then facies models should be modified to account for this variability.…”

“…Facies models for deltaic settings that incorporate ichnological data have been in existence for a number of years, but most are based on data from vertical facies successions such as those encountered in drillcores (e.g., Moslow and Pemberton 1988;MacEachern et al 2005;Sadeque et al 2007;Gani et al 2007;MacEachern and Bann 2008;Buatois et al 2012). There is, however, a paucity of datasets that document trace-fossil distributions across an entire ancient delta based upon three-dimensional exposure of the same sediment body, where direct lateral equivalence can be demonstrated (see Li et al 2011).…”

Palaeoenvironment of Eocene prodelta in Spitsbergen recorded by the trace fossilPhycosiphon incertum

“…Within deltaic systems, Phycosiphon has been observed mainly in sediments of the delta front, the transition zone between delta front and prodelta, and in the prodelta (i.e., Bann et al 2008;Buatois et al 2008;Carmona et al 2009;Buatois et al 2011;Weiguo et al 2011;Buatois et al 2012). Several papers addressed the immediate colonization of event beds by Phycosiphon trace-makers (Stow & Wetzel 1990;Goldring et al 1991;Wetzel & Balson 1992;de Gibert & Martinell 1998).…”