Strategies for assessing Early--Middle (Pliensbachian--Aalenian) Jurassic cyclochronologies

There are a number of fundamental problems in assessing the astronomically forced cyclostratigraphy of the Jurassic Period. First, Jurassic geochronology is not well constrained, due to a general scarcity of radiometric dates, inferior precision of the existing ones, and large inaccuracies in stratigraphic constraints. These problems are particularly troublesome in the Early to Middle Jurassic cyclic carbonates of the Colle di Sogno section in the Lombardy Pre–Alps, Italy. Second, Jurassic sedimentary formations have undergone significant diagenesis, and again, the Colle di Sogno carbonates, with their location on the southern edge of the Alpine fold belt, are no exception. This prevents the recovery of the primary geochemical proxies needed to evaluate the relationship between cyclic sedimentation and orbital forcing. Finally, even when a strong case can be made for orbitally forced cycles, a direct calibration between the Jurassic and existing theoretical orbital solutions is not yet possible. However, at Colle di Sogno new nannofossil biostratigraphy provides the opportunity to make significant headway in recognizing Jurassic signals consistent with orbital theory, and to propose cyclochronologic estimates for a portion of the Pliensbachian, and for the combined Toarcian–Aalenian Stages. We evaluate the presence of orbital signals in the sequence through the use of frequency modulation analysis of the cycles, as well as by spectral analysis of lithologic rank time–series reconstructions. The results show that the Pliensbachian Domaro Limestone contains a dual–component precession–like signal that undergoes frequency variation indicative of modulation by orbital eccentricity. The signal subsequently evolves into a single–component signal with frequency perturbations similar to those of the obliquity variation in the overlying Toarcian–Aalenian Sogno Formation. This switch in response to the fundamental orbital modes occurs during the deposition of the Lower Toarcian black shale, a time of significant global environmental change. The biostratigraphy at Colle di Sogno has well–defined Pliensbachian–Toarcian and Toarcian–Aalenian boundaries, allowing a cyclochronologic minimum estimate of 11.37 ±0.05 (1σ) Ma for the duration of the Toarcian–Aalenian interval (not accounting for stratigraphic and taxonomic noise). This compares well with the 13.1 ± 2.8 (1σ) Ma estimated for this same stratigraphic interval using the current radiometric geochronology. The underlying precession–forced Pliensbachian Domaro Limestone limestone–shale couplets suggest a time of formation for the sequence of least 5 Ma, with an age of onset in the jamesoni or polymorphus subzone.