Devonian lacustrine shore zone architecture: Giving perspective to cliff exposures using ground penetrating radar

Lake margin sedimentary systems have been the subject of only limited study. The orbitally controlled cyclic lacustrine successions of the Middle Old Red Sandstone of Northern Scotland contains repeated developments of shore zone sandstones and thus provides an ideal location for the study of these units. The cycles are on average 16-m-thick and comprise deep lake, perennial lake and playa facies. The shore zone facies reaches 2 to 3.5 m in thickness and is found within the playa facies. On the exposures, the shore zone facies seems to be aggrading with most of the layers apparently subhorizontal. However, this depositional environment ought to be relatively dynamic with sand body migration and erosions. In addition, along the coastal cliffs, the exposures are mainly on the same strike direction. To better understand the peculiar shore zone system, we carried out very high resolution sedimentary logging combined with high resolution GPR profiling (250 MHz). To efficiently tie the logs to the GPR data, the logs where realised on the cliff, using modified climbing equipment while GPR grids were shot directly on top. This configuration allowed to visualise in 3D the architecture of the shore zones and in combination with detailed sedimentology to unravel the subtle evidences of a dynamic shore zone environment.Detailed field observations are presented alongside ground penetrating radar data which has aided the three dimensional characterisation of these sand bodies.Loading and discrete channel forms are recognised in thin-bedded sandstones within the lower portion of the lake shore zone successions. Radar profiles provide evidence for an extensively developed sharp base to these units with some erosional features also recognised. Up-section the sandstone beds appear to become amalgamated forming subtle low angle accretionary bar complexes. These features are imaged well on the radar profiles where successive erosion/accretion can be recognised and their three dimensional form and distribution mapped. The orientation of these features is consistent with extensive palaeocurrent measurements from oscillation ripples. Further loaded sandstone beds and sand-filled shallow channel features overlie the bar forms. The channels are well imaged in the radar scans where their wider context can be gained.The shore zone sandstones overlie playa facies which contains abundant desiccation horizons, reflecting the most arid phase in the climatically controlled lacustrine cycle. As climatic conditions ameliorated the rejuvenation of fluvial systems resulted in the transport of sand out into the basin. Initial deposition was limited to intermittent events where sediment was laid down on a water saturated substrate. Some of these may have occurred subaqueously as small scale turbidity flows. High resolution fluctuations in lake level resulted in periodic short lived reworking events along the lake margin which produced amalgamated sands which formed low relief bars. Shore zone reworking is likely to have occurred over a wide area as the lake margin migrated back and forth, and gradually transgressed.