Sill stratigraphy and sedimentology in marine inundated basins, Sachs Harbour, NWT, Canada: Implications for sea-level reconstruction

This study focuses on the seismic interpretation and core-based sedimentology of shallow, coastal marine basins and their sills in the western Arctic to better constrain local relative sea level (RSL) records.;The study was carried out at Sachs Harbour, located on Banks Island in the western Canadian Arctic. The regional coastline is submergent with extensive evidence of drowned estuaries and inundated lakes. Sills associated with five basins were investigated using sediment coring techniques, airphoto analysis, and seismic profiling. Air photo analysis and field observations of two basins that have experienced relatively recent marine inundation suggest that initial marine penetration took place through the former lake outlets. It was hypothesized therefore that a similar inundation process took place in the deeper basins of Sachs Harbour.;Two buried channels and an erosion surface were identified on the seismic profiles of two sills and were subsequently targeted for coring. Recovered sediments (80-130 cm thickness) from channels were predominantly mud, which is interpreted to represent post-submergence marine sedimentation by suspension settling. A sediment core from one channel did not penetrate the channel bed, whereas in the other, an erosion surface was encountered at 87 cm core depth above laminated fine sand. Nearby on the same sill, the contact between laminated sand and overlying mud occurs at 22 cm core depth and is also interpreted to represent an erosional surface between in situ deglacial sediment and post-submergence marine deposits. Both erosional surfaces, when adjusted to modern sea level datum, occur at depths between 4.8 and 5 m and are considered to be good estimates of the former RSL position when marine inundation of the basin first took place.;Results of the study have confirmed that outlet channels are the most effective sill depth indicators; however, channel infilling and erosion may obscure the actual inundation elevations and thus provide false apparent depths. A combination of coring and seismic data is required to resolve this issue.;On transgressive coastlines, sea level rise can be measured using the inundation basin approach, which utilizes former coastal freshwater lake basins that have become inundated by marine water as sea level rises. The amount of sea level rise since inundation occurred is measured as the vertical distance between the lowest part of the former lake basin margin - the sill - and the current sea level position. While the approach has been typically applied to basins with rock-cored sills, this study investigates the opportunities and constraints presented by inundation basins with sediment-cored sills, which are more susceptible to post-inundation modification. Specifically, the study examines the processes acting on sediment-cored sills during transgression and their related sedimentary signatures.