Devonian lacustrine shore zone architecture imaged with ground penetrating radar

TY - ABST

T1 - Devonian lacustrine shore zone architecture imaged with ground penetrating radar

AU - Andrews, Steven D.

AU - Moreau, Julien

AU - Archer, Stuart

N1 - Conference code: 31

PY - 2015/6

Y1 - 2015/6

N2 - 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 contain repeated developments of shore zone sandstones and thus provides an ideal location for the study of these units. The cycles are on average 15 m thick and comprise deep lake, perennial lake, playa and shore zone facies (2 to 3.5 m in thickness). High resolution sedimentary logging and the construction of photo-panels were combined with high resolution GPR profiling (250 MHz). To ensure close ties between the sedimentary logs and the GPR data, the cliffs were accessed using rope access techniques while GPR grids were shot directly above. The profiles were shot mainly in the strike direction of what was thought to be the shore elongation every 5-10 m and every 20-30 m in the dip direction. Shore zone systems of 3 different sequences have been imaged for a total of 1155 m of GPR profile collected. This configuration has allowed 3D visualisation of the architecture of the shore zone systems and, in combination with detailed sedimentology, provided insights into the generation of the dynamic shore zone environments.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 that these units are sharp-based and contain some erosional features. 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 can be mapped. The orientation of these features are similar to extensive palaeocurrent measurements from oscillation ripples suggesting an alignment of the shore zone bars perpendicular to the prevailing wind direction. 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 contain 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, forming 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. Continued transgression forced fluvial systems back towards the basin margin.

AB - 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 contain repeated developments of shore zone sandstones and thus provides an ideal location for the study of these units. The cycles are on average 15 m thick and comprise deep lake, perennial lake, playa and shore zone facies (2 to 3.5 m in thickness). High resolution sedimentary logging and the construction of photo-panels were combined with high resolution GPR profiling (250 MHz). To ensure close ties between the sedimentary logs and the GPR data, the cliffs were accessed using rope access techniques while GPR grids were shot directly above. The profiles were shot mainly in the strike direction of what was thought to be the shore elongation every 5-10 m and every 20-30 m in the dip direction. Shore zone systems of 3 different sequences have been imaged for a total of 1155 m of GPR profile collected. This configuration has allowed 3D visualisation of the architecture of the shore zone systems and, in combination with detailed sedimentology, provided insights into the generation of the dynamic shore zone environments.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 that these units are sharp-based and contain some erosional features. 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 can be mapped. The orientation of these features are similar to extensive palaeocurrent measurements from oscillation ripples suggesting an alignment of the shore zone bars perpendicular to the prevailing wind direction. 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 contain 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, forming 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. Continued transgression forced fluvial systems back towards the basin margin.

M3 - Conference abstract for conference

T2 - 31st IAS Meeting of Sedimentology

Y2 - 22 June 2015 through 25 June 2015

ER -