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The AAPG/Datapages Combined Publications Database
A disconformity and overlying condensed bed at the Austin-Taylor contact were examined in eight outcrops over a distance (Austin-Dallas-Roxton) of 450 km. Results of the analysis incorporate information from petrologic, foraminiferal, and trace-fossil studies. Both disconformity and condensed bed formed in a continental shelf environment; shallowest water depths were at Austin. Proceeding northward from Austin, water depth (1) gradually increased to Temple; (2) increased greatly from Temple to Waco; and (3) decreased from Waco to Roxton, to depths similar to those at Temple. Assemblages of omission suite trace fossils that reflect these depth changes include: (1) large Thalassinoides, shallowest depths; (2) small Thalassinoides and Rhizocorallium, intermediate depths; and (3) small Thalassinoides, greatest depths. The condensed bed, in which thickness is as great as 18 cm, is characterized by abundant nodular phosphates that are synsedimentarily phosphatized steinkerns.
Halt of Austin sedimentation and formation of the disconformity was probably due to early Campanian regression, which caused: (1) shallowing; and (2) constriction of the southern aperture of the Western Interior seaway, which was directly northwest of the outcrop area. This constriction may have caused an increase in the velocity of currents through the aperture which, combined with shallowing, increased the energy level of bottom waters in the outcrop area and led to periods of erosion and minimal net sedimentation. Subsequent transgression caused an increase in water depth and a widening of the adjacent aperture. This may have resulted in a reduced energy level for bottom waters, which raised sedimentation rates and led to deposition of the condensed bed and overlying rocks of the T ylor group.
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