LESLIE, ALICK B., ALAN C. KENDALL, GILL M. HARWOOD and CAMERON LATIMER, School of Environmental Sciences, University of East Anglia, Norwich, UK

ABSTRACT: Conditions During Castile Formation Deposition, Delaware Basin - An Unresolved Problem

Interpretation of depositional environments of the lowermost Castile Formation (Anhydrite 1) is most critical, as conditions then were at their most extreme. Then, brine depths in the Delaware Basin were either deepest (deep-water evaporite hypothesis), or substantial drawdown occurred, with shallow-water brines across the floor of the near-empty basin. Three lines of evidence are important to any resolution of depositional environments of the Delaware Basin during the Ochoan:

1. Anhydrite 1 contains 12-15m thick sub-cycles, predominantly composed oflaminites, but each topped by poorly-bedded nodular/massive anhydrite. This latter anhydrite contains evidence for former bottom-growth gypsum crusts, and for displacive gypsum crystals added to the sediment during early diagenesis. Uppermost laminae also commonly display clastic fabrics;

2. Within Anhydrite 1, correlation of nodular/massive beds shows lateral continuity of the sub-cycles, indicating depositional changes were basin-wide. Thickening occurs due to turbidite addition, in the east of the basin, and active, syn-depositional tectonic control;

3. Not all sulfate has a depositional origin. In addition to displacive gypsum crystals, further diagenetic sulfate has been added between depositional laminae and as isolated nodules.

The origin of the Castile depositional cycles is thus still unresolved. Whereas sedimentological evidence from the tops of the sub-cycles argues for existence of basin-wide shallow (?50m) brine depths, deeper water conditions better explain the lateral continuity of the sub-cycles and their thickening by turbidite addition. Local post-depositional sulfate addition and gypsum dehydration to anhydrite obscure earlier fabrics.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.