Trace fossils in Cretaceous-Tertiary (KT) boundary beds in northeastern Mexico: implications for sedimentation during the KT boundary event

Cretaceous-Tertiary (KT) sequences in Nuevo León and Tamaulipas, Mexico, contain trace fossils that shed important light on the nature and duration of deposition of the KT boundary strata in that region. The KT clastic sequence in northeastern Mexico typically is divided into three distinct sedimentary units which represent distinctly different depositional events. Unit I is an unbioturbated, laminated deposit of alternating smectite grains and calcite spherules. Unit II is a sandstone that is mostly unbioturbated, but a few spherule-filled burrows occur near the base of the unit. Several burrows were truncated by overlying sand layers within Unit II, indicating that they were excavated following deposition of the first sand layers and then filled with spherules, scoured, and overlain by more Unit II sand. Unit III consists of alternating sandstone, siltstone, and shale that contain abundant trace fossils, including Chondrites, Ophiomorpha, Planolites, and Zoophycos. The nature of the trace fossil occurrences attest to at least three successive colonization episodes of the accreting substrate. The sandstone beds of Unit III were deposited episodically, and burrowing occurred during the period of deposition, not after deposition had ceased. The burrows were filled with late Cretaceous sediment. Trace fossil evidence indicates, therefore, that the entire KT clastic sequence must have been deposited over a long period of time. If the spherules in Unit I are material derived from an extraterrestrial impact, that impact must have predated the extinction of Cretaceous plankton by a significant time interval, which is represented by the periods of deposition of Units II and III. The ichnologic information indicates episodic deposition of Units II and III over an extended time period. Thus, the event that produced the calcite spherules in Unit I is not directly related to the Cretaceous plankton extinctions at the KT boundary, which occur at the top of Unit III.