Marginal marine clastic Tidal facies
The sandstone body is tabular, about 5.5 m thick and can be traced along the cliff over a distance of over 500 m of gently SE-dipping strata. It is overlain and underlain by dark mudrocks and the whole succession contains mainly brackish water fauna. The sandstone is a complex, heterolithic body composed of a mixture of sandy and muddy facies that show evidence of both wave-generated and current-formed sedimentary structures. Wavy and lenticular bedding are indicators of strong wave influence in a setting where there were variable proportions of sand and mud supply. Cross-bedding in thicker sandstone bed shows a wide range of palaeocurrent directions, with evident herring-bone cross stratification in places. The succession is interpreted (Yoshida et al 2001) as a series of three stacked tidal sandbars. The proportion of sand in the succession increases suggesting increased sand supply through time. The setting is considered to be a tidal estuary, although there is no clear evidence of confinement of the sandstone within a drowned valley at a river mouth. This sandstone body illustrates the internal complexity and heterogeneity of sandstones deposited in tidal settings. The variable proportions of mud and the various sedimentary structures present barriers to fluid flow to be taken into account when modelling a tidal sandstone reservoir of this type (see Jackson et al 2005)
Viewing the Image
The small-scale wavy and lenticular bedding is not easily picked out on an image at this scale. However, the larger-scale lenticular shape of the tidal sandstone bodies can be identified by tracing individual sandstone units across the cliff face. These thicker sandstone bodies have the best reservoir properties but are variable in thickness and proportion of mud over distances of tens to hundreds of metres.