Astronomical Tuning of the Aptian Stage (Piobbico Core, Italy)
A high-resolution greyscale series of the pelagic Fucoid Marls (Piobbico core, Central Italy) shows pervasive lithological rhythms throughout the Aptian. We recognize a hierarchy of meter/decimeter-scale cycles characterizing these rhythms: ~4 m, ~1 m, ~0.4 m, ~0.2 m and 0.12 m cycles. These cycles respectively correspond to ~2 Myr, 405 kyr, 200 kyr, ~100 kyr and ~ 41 kyr periods when calibrating the ~1 m cycles to the 405 kyr long-eccentricity. High-frequency precessional cycles at the cm scale are only occasionally resolved when sedimentation rates are relatively high. This astronomical calibration allows a high-resolution estimate of the duration of the Aptian stage. The timing and duration of the Aptian remains uncertain and has been the subject of controversy for many years. Gradstein et al. (2004) suggested that the base and top of the Aptian stage are 125.0±1.0 Ma and 112.0±1.0 Ma, respectively. Selby et al. (2008) give the age of the Aptian-Albian boundary as 113.1 ±0.3 Ma through application of Re-Os and 238U/206Pb dating techniques. Using 40Ar/39Ar geochronology, Chambers et al. (2004) obtained a date of 123.5 ±1.8 Ma for the basal Aptian; He et al. (2008) estimated the age of the Barremian-Aptian boundary as 121.2 ± 0.5 Ma. These estimates collectively indicate a duration for the Aptian Stage that ranges from 9 to 13 Myr. Moreover, recent K-Ar dating of glauconite suggests a stage duration as brief as 6.8 Myr (Fiet et al., 2006). Our astronomical tuning of the Piobbico core cannot establish absolute age, but it indicates a minimum duration of 12.6 Myr for the Aptian stage. The estimated durations are 1.453 Myr, 4.256 Myr, 0.914 Myr, 3.085 Myr, 0.716 Myr, 0.818 Myr, 2.116 Myr for the H. planispira, G. bejaouaensis, H. trocoidea, G. algerianus, G. ferreolensis, L. Cabri, G. blowi planktonic foraminifer zones, respectively. The estimated durations are 2.256 Myr, 7.419 Myr, 2.659 Myr for the N. regularis, P. angustus, C. litterarius calcareous nannofossil zones, respectively. There is also evidence for several disrupted intervals or hiatuses along the core, which could signify an even longer duration for the stage.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009