Rainer Zühlke¹, Thilo Bechstädt¹

(1) Institute of Geology and Palaeontology, University of Heidelberg, Heidelberg, Germany

ABSTRACT: Sub-Milankovitch and Milankovitch cyclicity in a model carbonate platform - Latemar, Triassic, Italy

The platform interior of the Latemar shows a distinct bundling of shallowing-upward microcycles into thinning-upward macrocycles. Based on 5:1 bundling patterns previous researchers assumed a precessional forcing for microcycles and an eccentricity forcing for macrocycles. New high-resolution 3D cyclostratigraphic data, chrono-/biostratigraphic data and spectral analyses indicate different controls of the Latemar cyclicity. The total number of cycles is 626-745, considering different locations on the platform and minimum/maximum numbers of cycles. Ammonoid faunas indicate that the cyclic succession was mainly deposited in a single biozone (Secedensis Zone). The time interval bounded by two age-dated volcaniclastic layers is 0.5-2.1 My. Both layers enclose 400-440 cycles. Consequently, individual shallowing-upward microcycles have average durations of only 1.1-4.8 ky. This clearly indicates sub-Milankovitch forcing. Different spectral analyses methods applied to the Latemar series show highly significant ratios of 1:4 to 1:6, 1:10, 1:20 to 1:27 and 1:100. Ratios are largely constant over the cyclic succession. If D t is set to 4.2 ky, Triassic orbital parameters (ratios, time intervals) are present at highly significant frequencies. In this model, the macrocycles in the Latemar represent precessional forcing, 1:10 bundlings obliquity forcing and 1:20 to 1:27 bundlings short eccentricity forcing. In general, 5:1 bundling between hierarchic cycles does not necessarily indicate precessional and eccentricity forcing. Genetic cyclostratigraphic models should be time-calibrated by chrono- and biostratigraphic data, i.e. model-independant data.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado