The Rozel Conglomerate Formation

We drove north to Belval Cove where we met Dr Arthur Hill, Mister Wacky Wellies himself. He was very amusing and was always telling stories, which we were never sure were totally bona fide! Belval Cove is on the northern side of St. Catherine's Bay, near the NE corner of Jersey which is composed of the Rozel Conglomerate. The purple book says that a dyke which intruded the Rozel was dated at 435 - 13Ma so this sets the minimum age. The conglomerate overlies the Bouley Rhyolite Formation which outcrops to the south and west and is therefore younger; this makes the Rozel the youngest major solid deposit on Jersey. Unfortunately, although it has ripplemarks and raindrop inprints preserved in the shales, the Rozel contains no body fossils and nothing useful for dating purposes.

We scrambled down to see surfaces exposed in sides of several gullies which Arthur said might be faults. There was an obvious dip of about 60 degrees inland; we played "spot the imbrication" with pebbles to try to work out the palaeocurrent flow and it also seemed to be from inland. The purple book says (Chapter 4 page 41) that the underlying rhyolite was eroded and gullied and that the conglomerate dips against this palaeosurface, filling up the hollows. The purple book confirmed our observations, stating that the direction of flow had been from the north mostly, which was inland at Belval Cove.

The conglomerate was full of angular clasts which formed fining-up sequences to red shale in the lower half. The upper half became coarser and greyer in colour. In between was a long lens-shape which was cemented with grey instead of red cement. Arthur said this showed that the deposit was from fast-flowing water (hence the lens - it was a shallow braided stream channel) with transport for a short distance and fast flow rates which declined rapidly (angular clasts, imbrication, fining-up) in a dry oxidising desert environment (red cement and iron staining of clasts) so the Rozel Conglomerate was a wadi deposit with flow rates increasing overall (no shales in upper half of the exposure). The purple book says that 2m diameter boulders occur but the largest we saw were about head-sized. It identifies clasts of acid plutonic rocks which show some rounding, angular metasediments and volcanics. The latter were eroded from the Brioverian Jersey Shale and Jersey Volcanic Group but the subrounded pebbles do not match local rocks and have not come from Jersey. There are no clasts of the nearby Cadomian granites which outcrop to the NW so these could not have been unroofed until after the Rozel Conglomerate had been deposited. The Green Island SE gabbros/diorites and granites would have been carried away from Belval Cove by a southward flowing current so we cannot know whether or not they were exposed at that time.

Heidi Barnes