Analysis of fluvial-tidal transition zones using sedimentology, geochemistry, palynology, and modern analogs
Creators: name: Northrup, Dustin Shawn
Publisher: name: University of Iowa
Year: 2023

description: Fluvial-tidal transition zones (FTTZ) represent one of the most complex depositional systems within ancient strata due to their dynamic nature. The Lower Cherokee Group in southeastern Iowa records a previously undocumented Lower-Middle Pennsylvanian aged FTTZ. We use a combination of detailed core descriptions and geochemical analysis obtained with a handheld X-ray Fluorescence spectrometer (pXRF) to describe 10 distinct geochemical zones that can be identified across the study area. The geochemical data provide a framework within which palynological and RockEval pyroloysis data are integrated to develop a biogeochemical framework for the Lower Cherokee Group. The results suggest that the study area was dominated by lycopod forest swamps throughout the mid-late Atokan. Sedimentary structures, trace fossils, and palynology suggest intermittent marine conditions within the study area during deposition of the Kilbourn Formation, but that relative sea level fall persisted throughout much of the deposition of the Kalo and Floris formations. The Floris Formation coincides with the arrival of continental-scale fluvial systems to the region originating from the Alleghenian Orogeny to the east as the basin bounding arches separating the Forest City Basin, Illinois Basin, Michigan Basin, and Appalachian Basin were overtopped by sedimentation. This was followed by the first full marine transgression coinciding with deposition of the Oakley Shale. Abundant charcoal throughout the Lower Cherokee Group suggests frequent wildfires in the region during Early-Middle Pennsylvanian time.
description: Within ancient FTTZs it is challenging to differentiate sedimentary strata deposited under marine, marginal marine brackish-water, or freshwater conditions. Presented here are sediment geochemistry transects from along the length of two modern fluvial-tidal transition zones: the Ogeechee River and estuary in Georgia, USA and the Petitcodiac River and estuary in New Brunswick, Canada. These data are used to test the effectiveness of two previously proposed paleosalinity proxies and explore for other relationships between sediment geochemistry and salinity. Our results indicate that Sr/Ba is moderately effective but strongly dependent on grainsize, whereas S/TOC is ineffective due to the various controls exerted on S and TOC concentrations in the depositional environment. Multivariate discriminate analysis (MDA) of the full geochemical dataset demonstrates a strong relationship between grainsize and geochemistry. as well as the specific river and the sediment geochemistry, but no relationship between sediment geochemistry and water salinity. Principal component analysis (PCA) does not identify any elements that strongly covary with salinity. We suggest that paleosalinity proxies may not work across all depositional systems due to geochemical controls unique to each region, such as grainsize, mineralogy, and sediment provenance. We recommend a very measured and reserved approach to utilizing paleosalinity proxies that should include multiple proxies when possible. This also necessitates understanding the unique controls exerted on each system before developing a localized paleosalinity indicator.