Upware revisited – a fresh look at the ‘coprolites’ of the Lower Greensand (Aptian, Early Cretaceous) in Cambridgeshire

A temporary Lower Greensand section and its unconformity with underlying Upware Limestone and Ampthill Clay members (West Walton Formation, Oxfordian, Jurassic) is discussed. For the first time since the cease of commercial ‘coprolite’ workings here in the late 19th century, phosphate-rich Lower Greensand is well exposed. At the base of the Lower Greensand rounded blocks of Upware Limestone are heavily bored by flask-shaped Gastrochaenolites; locally fresh-shelled brachiopods are common; higher-up, Entolium and exogyrine oysters occur; the ammonite Ancyloceras hillsi from earlier collections indicates a Late Aptian age for these indigenous faunas. The lower 2 m of the Lower Greensand contain three beds rich in reworked phosphatised clasts, many with common Late Jurassic and Early Cretaceous fossil moulds. The clasts vary from fine grained mudrock to very coarse sandstone, and are heavily phosphatised, often rounded and blackened, hence their naming as ‘coprolites’ by early commercial exploiters. True coprolites are rare. The phosphatised faunas so far indicate: Oxfordian: Amoeboceras; Kimmeridgian-Volgian: perisphinctids, Pleuromya; Mid Volgian-Ryazanian: ?Subcraspedites, Dicranodonta, Myophorella, Lyapinella, Rouillieria; Early Aptian: Deshayesites; Mid Aptian: Cheloniceras. Pleistocene reworking of the Lower Greensand gave rise to poorly bedded sands and pebble beds resulting from collapse of permafrost-bound cliff into ephemeral Fenland flood lakes.