Inside Mid-Triassic fluvial deposits: a legacy of mining sediment-hosted ore bodies in Cheshire, UK

In the Cheshire Basin the Mid-Triassic Helsby Sandstone Formation includes deposits of fluvial and aeolian origin. Sedimentological details of these deposits are commonly obscured in weathered outcrops but can be studied in unweathered sections in disused mines at the Alderley Edge Geological Site of Special Scientific Interest in northeast Cheshire. Fining-upward cycles seen in the fluvial units comprise channel-fill deposits that rest on an erosion surface, grade upwards from conglomerate to fine sandstone and are overlain by mudstones that underwent erosion at the initiation of the next cycle. The mudstone components of some cycles remain in parts of the mines but others were eroded completely and are now represented by concentrations of intraclasts in the succeeding cycle. Mudstone beds and those containing intraclasts formed complete and partial barriers to fluid movement respectively. Their effect on the migration and accumulation of intrastratal fluids is reflected by the form and disposition of voids left by the mining of ore bodies hosted by this facies. A corollary is that if the voids are interpreted as representing hydrocarbon accumulations, rather than ore bodies, reservoir situations analogous to those in the Helsby Sandstone in comparable structures in the East Irish Sea Basin may be viewed from a unique internal perspective.