Sediment textural characteristics of the Ravenglass Estuary; Development of a method to predict palaeo sub-depositional environments from estuary core samples

Abstract

Here we present a new way to automatically classify the exact sub-environment of deposition of sediment from estuarine sediment cores. It can be challenging to define the exact sub-environment of deposition in core as sediment of a given appearance, or facies, can be found in multiple settings. This issue is important given that petrophysical, geomechanical and reservoir quality properties of sedimentary rocks are typically strongly influenced by the specific sub-environment of deposition. Here, using a ten-fold classification of depositional sub-environments, we have determined the sub-environments of 482 sample sites from the Ravenglass Estuary, in NW England, UK. We then analysed the textural characteristics of each of these samples using laser particle size analysis. A novel automatic textural classification scheme was then developed using a combination of visual discrimination of gravel and vegetated surfaces, principal component analysis and a recursive partitioning routine (RPART) in Rstudio. The new automatic textural classification scheme can resolve eight of the ten sub-environments of deposition: gravel beds, salt-marsh, mud flat, mixed flat, sand flat, tidal inlet, combined south foreshore/ebb-tidal delta and combined tidal inlet/north foreshore. Our scheme cannot differentiate the spatially adjacent tidal inlet and north foreshore sediments as they are texturally identical. Similarly, the scheme cannot differentiate the spatially adjacent ebb-tidal delta and southern foreshore sediments as they also are texturally identical. We have applied our surface-calibrated method to a 3 m Holocene core drilled through fine-grained surface sand flats into interbedded fine- and coarse-grained sands in the Ravenglass Estuary and successfully defined palaeo sub-environments of deposition. Our automatic approach to the definition of palaeo-environment of deposition supersedes a simple lithofacies-based approach for the Ravenglass Holocene core, as we can define, cm-by-cm, how the exact estuarine sub-environments evolved over the last 10,000 years. This approach could also be applied to other modern estuaries and could be trialled for use with ancient and deeply buried sedimentary rocks deposited in equivalent marginal marine estuarine environment.