BIOSTRATIGRAPHY AND IMPACT STRATIGRAPHY (Invited Presentation)

Today it is virtually unthinkable to solve complex geologic problems without biostratigraphy based on microfossils, whether for relative age control or environmental proxies. This is amply demonstrated for the end-Cretaceous (KTB) mass extinction where radiometric dating cannot decipher the order of events because they fall within dating error limits. But relative age dating is uniquely positioned to identify any ordered sequence of closely spaced events over a short time interval. The KTB mass extinction is one of the most easily recognized boundary events in geological history with over 300 sequences identified globally and independently verified based on the same set of five criteria originally proposed by the ICS working group during the late 1980s–1990s (mass extinction, evolution of first Danian species, d¹³C shift, Ir anomaly, clay layer) [1]. Despite this successful track record, placement of the KTB is contentious in areas with Chicxulub impact spherules because they are found below, at and above the KTB as a result of reworking after the primary fallout deposit [2].

As a result, biostratigraphy has come under attack by what could be called impact exuberance driven by the belief that the Chicxulub impact caused the KTB mass extinction and therefore must be KTB in age. Accordingly, some micropaleontologists proposed to redefine the KTB solely based on the ‘‘Ir anomaly associated with a major extinction horizon’’ [3], or any impact signals “in order to solve problems with correlation” [4]. But rather than solving problems this has introduced circular reasoning as the defining criteria: Chicxulub caused the mass extinction, therefore Chicxulub is KTB in age. This leaves no room for determining the actual age of the Chicxulub impact. Because impact signals are subject to erosion and reworking as evident in all Chicxulub impact deposits [5,6], determining the age relative to the KTB necessarily depends on biostratigraphy to sort the relative sequence of events [7]. Impact stratigraphy cannot stand on its own without the relative dating of biostratigraphy.

[1] Keller, 2011, SEPM 100, 23-42. [2] Keller, 2011, SEPM 100, 7-22. [3] Gradstein et al., 2004,[4] Molina et al., 2006, Episodes 29, 263–273. [5] Keller et al., 2009, JGS London, 393-411. [6] Keller et al., 2013, G. Mag. 885-907. [7] Keller, 2008, SP 437, 147–178.