Phosphorite in Eastern Llano Uplift of Central Texas

A deposit of phosphorite, estimated to be 11 feet thick, was found March 26, 1952, while the writer was mapping ancient sink fillings and collapse structures within the outcrop area of the Honeycut formation south of Marble Falls, Texas. The phosphorite is exposed in a road material pit on the eastern side of a Carboniferous outlier…It rests on shale typical of that in the Barnett formation of Mississippian age and is beneath spiculitic limestone at the base of the Pennsylvanian Marble Falls limestone. Geiger counter measurements, both in the road material pit and on the opposite side of the hill at the same stratigraphic level, gave values of about 100 counts per minute, the background count being about 40 per minute. Another outlier about 500 feet to the northeast possibly contains a similar sequence. Limestone of the type found in the phosphorite in the road material pit occurs as float about this outlier indicating that the interbedded phosphorite may also be present. A Geiger counter examination failed to reveal any abnormal readings, but the phosphorite may be present and masked by soil. These outliers probably are in structural sinks because evidence of collapse is well displayed in the road material pit where both the Honeycut and Barnett formations dip steeply inward. The position of the phosphorite suggests that in age it is between the Barnett formation and the Marble Falls limestone. From stratigraphic position alone it could be a unit of the Barnett formation not hitherto recognized or a basal unit of the Marble Falls limestone. However, conodonts from a limestone bed in the phosphorite and from phosphorite adhering to the bed have been identified by Dr. W. H. Hass of the United States Geological Survey as predominantly Lower Pennsylvanian in age, thus eliminating the possibility that the phosphorite is a member of the Barnett formation.