Abstract

Mr R. Mortimore said: Although the iron content of the Yorkshire Basin Cleveland ironstones probably represents a quite different chemical environment from the central England Platform phosphates, there seems to be a possible sedimentological similarity. The oolitic ironstones appear to have formed on diastem hardgrounds and the oolites are found infilling burrow systems and scattered in the overlying sediment much as the pelleted phosphate occurs in the Anglo-Paris Basin Chalk. Evidence of cross-bedding and intraformational unconformity indicate erosional and high energy conditions in the ironstones. The possibility of ironstone ooliths being swept off these hardgrounds into deeper muds not affected by sea-floor hardening is evident.

If a similar sedimentological regime existed on the platform SE of the outcrop of the central England Upper Lias, it could have provided a diastem, shoaling environment for the formation of ooliths and pisoliths. Reworking and transportation of the ooliths into deeper water muds banking up against the platform would be a simple matter.

It is then a question of the geochemical environment, deciding which mineralizing process will develop, iron or phosphate. In this situation can you see any significance between the apparent mutually exclusive pairs glauconite-chamosite and francolite-siderite, and could you see the Cleveland Ironstone hardgrounds as providing a clue to the sedimentary environment of the central England Upper Lias phosphates?

In reply, the Authors state that the source area of the Upper Liassic phosphatic öoids has not been located and so we are unable to compare it with the postulated depositional environment of

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