Internal Geometry of Ancient Tidal Bedforms Revealed Using Ground Penetrating Radar

Abstract

This paper represents the results of a ground penetrating radar (GPR) survey in the Woburn Sands Formation, Lower Greensand Group near Leighton Buzzard. Almost 4 km of survey lines were taken to investigate the internal structure of the sands. The radar survey achieved a resolution on the order of tens of centimetres with a penetration up to 10 m. It revealed a variety of horizontal, dipping and curved reflectors. The reflections are described in terms of radar facies by identifying repeated packages of reflections with similar character and geometry, following the approach for seismic facies analysis. On this basis nine different radar facies have been identified: (i) large sets of planar cross‐stratification; (ii) cross‐strata with asymptotic toesets; (iii) sets of trough cross‐stratification; (iv) cosets of cross‐stratification; (v) tidal bundles; (vi) clay drapes; (vii) erosional scours; (viii) diagenetic concretions or voids; and (ix) bioturbated sands. A three‐dimensional reconstruction revealed large bedforms with troughs resembling very large, curved‐crested dunes. The troughs are up to 9 m deep and up to 60 m wide. The bedforms are strongly asymmetrical with steep lee slopes and low‐angle stoss slopes, indicating a strong time‐velocity asymmetry in the tidal currents. The internal erosion and reactivation surfaces are consistent with type II sandwaves of Allen (1980). Most of the bedforms observed appear to be truncated, suggesting that their original heights were substantially greater. Lateral changes in reflection character indicate lateral changes in bedform type and sedimentary structures. These are interpreted to have been formed by bedforms in the classes IA, IIA, IIIA and IVA of Allen (1980). The lateral variations in facies suggest that the Woburn Sands Formation is a complex sheet deposit consisting of very large curved‐crested dunes and smaller superimposed dunes. The interpretations are compared with local outcrops in working sand pits.