ICHNOLOGY AND SEDIMENTOLOGY OF MIOCENE SHALLOW- TO MARGINAL-MARINE DEPOSITS OF NORTHWESTERN VENEZUELA: ANIMAL-SUBSTRATE INTERACTIONS IN MODERN AND ANCIENT WAVE INFLUENCED COASTAL SYSTEMS
The study of trace fossils is of paramount importance to recognize different depositional environments within coastal systems, which usually display a mosaic of shorefaces, deltas, lagoons and bays along the same coast. However, the ichnology of deltas is comparatively less understood than that of non-deltaic shorelines and estuaries, and most research has been focused on subtropical to temperate paleolatitude deltas of the Cretaceous Seaway of North America, with only a few studies from the tropics. Ichnologic and sedimentologic studies in wave-influenced coastal systems of the middle- upper Miocene Urumaco Formation, in northwestern Venezuela identified three main sedimentary environments: (1) Basinal and low-energy shallow-marine deposits, which are dominant during transgressions, are characterized by high ichnodiversity and intense bioturbation, recording a fully marine expression of the Cruziana Ichnofacies. (2) Regressive wave-influenced shallow-marine and deltaic deposits contain sporadically distributed trace fossils, display reduced ichnodiversity and show variable bioturbation intensities, illustrating a depauperate Cruziana Ichnofacies. However, wave-dominated deltaic deposits show higher ichnodiversity and degrees of bioturbation than river-dominated deltas, reflecting less stressed conditions associated with strandplains, characterized by the Cruziana and Skolithos Ichnofacies. (3) Delta-plain and marginal-marine environments contain brackish-water trace-fossil assemblages in interdistributary bays and lagoons, whereas upper delta-plain deposits encompass immature paleosols, overbank deposits, and highly incised fluvial channels, which are essentially unbioturbated. This study identified a complex stratigraphy of high-frequency transgressive- regressive cycles, in which the progradation of an asymmetric wave-dominated delta complex, alternated with the formation of barrier islands and lagoons during transgressions. The Glossifungites Ichnofacies delineates discontinuity surfaces and helps to establish an early brackish-water transgression associated with a lagoon system, and a late and more laterally extensive fully marine transgression represented by coquinas and associated wave ravinement surfaces. The dominance and high diversity of crustacean burrows is an ichnologic signature of the tropics, including monospecific associations of Sinusichnus in lagoon deposits. An extensive coastal wetland system developed in western Venezuela and extended south into the Amazon/Orinoco region through lowland passages across the raising Merida Andes during periods of high sea level, explaining the biogeographic connection of the Urumaco vertebrate fauna with the proto-Orinoco river. The trace fossil Macaronichnus is reported for the first time from low-latitude, tropical settings based on its occurrence in nearshore deposits in the upper Oligocene-lower Miocene Naricual Formation, and the middle-upper Miocene Urumaco Formation in Northern Venezuela. Macaronichnus is an intrastratal trace fossil attributed to the deposit-feeding of worms in high- energy, sandy shallow-marine environments. The majority of its occurrences are from Mesozoic to Cenozoic high- to mid-latitude shorelines. The opheliid polychaetes Ophelia limacina and Euzonus mucranata make identical structures to those described from the fossil record. Macaronichnus shows a similar geographical and environmental distribution in the fossil record to that of its modern producers, and has been proposed as an indicator of high- to mid-latitudes. Accordingly, its presence in the Neogene of Venezuela is highly anomalous and seems to challenge its paleoclimatic value. However, this occurrence may be related to seasonal coastal upwelling of nutrient-rich cold waters. Such oceanographic conditions were prevalent in the northern coasts of South America, at least from the Late Oligocene to the Early Pliocene prior to the final closure of the straits of Panama. This study underscores the value of Macaronichnus because its presence in the tropics may indicate upwelling conditions, providing high-resolution information in paleoceanographic reconstructions. To test this hypothesis that was solely based on the fossil record, several trenches and sediment peels were made in two high-energy sand beaches, with different oceanographic conditions, along both the Pacific and Caribbean coast of the Central American Isthmus. Macaronichnus were found only on the highly productive waters of the Pacific coast of Costa Rica in connection with upwelling, while they were absent from the oligotrophic waters of the Caribbean coast of Panama. This finding demonstrates that sometimes the past may be a key to the present, providing an example of reverse uniformitarianism. The results of this study underscore the ichnologic signatures of tropical marginal-marine environments, further exploring the paleoclimatic dimension of biogenic structures. The dominance and high diversity of deep-tier crustacean burrows in the Urumaco Formation is characteristic of the tropics, whereas the occurrence of Macaronichnus in the Miocene of Venezuela and recent sands of the Pacific coast of Costa Rica, in connection with upwelling, enhances the value of this ichnogenus as a new paleoceanographic tool. Articulation of neoichnologic studies with exploration of the trace-fossil record of Cenozoic successions allows a more refined characterization of marginal-marine coastal deposits and biotas in the Caribbean region.
Ichnology, Sedimentology, Coastal Systems, Urumaco Formation, Venezuela
Doctor of Philosophy (Ph.D.)