The allocyclic interpretation of the ‘Latemar Cycles’ (Middle Triassic, the Dolomites, Italy) and implications for high-frequency cyclostratigraphic forcing - Perspectives in Carbonate Geology - page 210

The allocyclic interpretation of the 'Latemar Cycles'
(Middle Triassic, the Dolomites, Italy) and implications for
high-frequency cyclostratigraphic forcing

ROB M. FORKNER *1 , LINDA A. HINNOV ‡ , ROBERT K. GOLDHAMMER *† and
LAURIE A. HARDIE ‡
* University of Texas at Austin, Jackson School of Geosciences, Department of Geological Sciences
1 University Station C1100, Austin, Texas, 78712, USA (E-mail: [e-mail])
‡ Department of Earth and Planetary Sciences, Olin Hall, 3400 North Charles Street, Johns Hopkins University,
Baltimore, Maryland 21218, USA

ABSTRACT

The Middle Triassic Latemar Platform is an approximately 5 km wide, 700 m thick
isolated carbonate platform succession containing over 500 cycles (<1 m average
thickness), which have been attributed to allocyclic forcing by Milankovitch to sub-
Milankovitch cycle driven composite eustasy. These interpretations are based on the
facies composition of the cycles (thicker subtidal units overlain abruptly by thin, cen-
timetre-scale, subaerial caps), the 5:1 bundling of the 'fundamental' metre-scale cycles
into lower-frequency 'megacycles', and spectral analyses of thickness and rank series
showing conspicuous matches to predicted Milankovitch periodicities. The infl uence
of periodic Milankovitch cycle-driven composite eustasy on the development of the
Latemar cyclic succession has been questioned based on the dating of ashfall tuffs,
palaeomagnetic analysis of the platform, and correlation to biostratigraphic markers
in nearby basinal deposits. In order to test the interpretation of allocyclic forcing for
Latemar cycles, the cyclic succession preserved at Mendola Pass (located 30 km north-
west of Latemar Platform) was investigated where Latemar-equivalent cyclic platform
interior strata are well exposed. The Mendola cycles (average 0.70 m per cycle) are
also bundled into upward-thinning packages with an approximate 5:1 ratio. However,
unlike the subtidal deposits immediately overlain by vadose diagenetic caps found
in the Latemar succession, Mendola cycles consist of a mud-rich subtidal unit grada-
tionally overlain by a cryptomicrobial (peritidal) laminite cap. A measured section
from Mendola Pass of 36 cycles correlates biostratigraphically and statistically to a
unique interval within the Latemar succession. Although laminite-capped cycles are
often attributed to autocyclicity, the similarity of the stacking patterns of the Mendola
cycles to those of Latemar support an allocyclic interpretation. In addition, deposi-
tional rates calculated from dated Holocene shallow-water carbonate facies equivalent
to those at Latemar and Mendola Pass are shown to be consistent with Milankovitch
or multimillennial periodicities, rather than millennial (1 kyr) cycle periods. Finally,
the question of whether comparative sedimentology can be relied upon for the inter-
pretation of relative cycle duration is considered, or if comparative sedimentology
has reached its useful limit for identifying facies and depositional environments with
respect to the Middle Triassic. It is concluded that statistical and biostratigraphic cor-
relations, in addition to comparative sedimentology, indicate that the Latemar and
Mendola cycles were deposited under the control of an allogenic forcing mecha-
nism, and that this mechanism generated depositional cycles with multimillennial
individual periodicities.

Keywords Middle Triassic carbonates, cyclostratigraphy, high-frequency eustasy.

INTRODUCTION: THE PROBLEM † Deceased.

1 Present address: Shell International Exploration and
Production B.V. Kessler Park 1, 2288 GS Rijswijk, The
Netherlands.

Investigation of the cyclic succession of platform
carbonate deposits at Latemar began over 20 years