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Fusulinid foraminifers and ammonoid cephalopods occur in mutually exclusive positions in the regressive portions of Late Pennsylvanian cycles exposed on the eastern shelf of the Midland basin in north Texas. Ammonoids occur consistently in deeper water parts of the cycles than fusulinids do, and are common in phosphate nodule-bearing dark shales that occur immediately over transgressive limestones. Their abundance rapidly decreases upward in the overlying gray shales of the regressive sequence. Fusulinids are absent from ammonoid-rich dark shales, and first appear in overlying gray shales where ammonoids are rare and of small size. Fusulinids found in these strata are small juveniles, and not transported. Full-size fusulinids occur in overlying gray shales containing high diversity biotas. Concentrations of unabraded fusulinids occur in shales located at the margins of limestones and sand sheets in the mid portions of the regressive sequences, and are formed by current winnowing of mud or accumulation in areas of reduced sedimentation. Fusulinids are rare in unequivocal shoal-water deposits, occurring mostly as transported, worn, and fragmented individuals in sandstones capping regressive sequences. These occurrences suggest that fusulinids are indicators of moderate depth, mid-shelf environments in Late Pennsylvanian north Texas sequences, while ammonoids are indicators of deeper water environments. The lack of mature fusulinids in deeper water deposits containing small juveniles lends support to the conclusion that fusulinids contained symbiotic microal ae, and were restricted in life occurrence to the photic zone.
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