Abstract
AbstractThe Triassic Dachstein platform limestone at Steinernes Meer, Salzburg, Austria, includes 611 m of limestone with 222 peritidal cycles overlain by 273 m of subtidal, non‐cyclic and weakly cyclic limestone. Cycle patterns include both shoaling and deepening upward, symmetrical, truncated, and couplets without a depth vector. Beds are laterally discontinuous, and cycle bounding surfaces are laterally variable in the studied strata. Of 558 subtidal and intertidal beds measured, 121 (21.7%) disappear laterally. An additional 73 beds (13.1%) show significant (>10%) lateral variations in thickness. Mean thickness variation is 49%. Both lateral variations and terminations appear to lack a spatial vector. Disappearances toward the inferred platform interior (west) total 10% of the beds. East, toward the platform margin, 11.6% of the beds disappear. Thickness changes occur in 6.5% of beds in each direction. The lack of lateral continuity of beds precludes a simple allocyclic forcing model and is consistent with a non‐eustatic component to stratification. Erosion of intertidal intervals is the process that can be most readily documented. Non‐uniform rates of production, transport and distribution of sediments, superposed on stratigraphic sequences driven by eustasy, probably also contributed to the complex cycle patterns recorded in the Dachstein. Such composites of autocyclic and extrabasinal factors should not be uncritically interpreted as exclusive records of orbital forcing. Lateral discontinuities and thickness variations would also produce inaccuracies in spectral analysis of thickness patterns, typically conducted in search of ‘Milankovich frequencies’, as well as in construction of Fischer plots to analyse long‐period oscillations in accommodation. Any section subjected to cycle analysis should be examined for lateral changes, to the extent permitted by the exposures, in order to produce the most complete (composite) section possible.
Highlights
Shallow‐water platform carbonates of all ages commonly show repetitive, cyclic patterns
The Triassic Dachstein platform limestone at Steinernes Meer, Salzburg, Austria, includes 611 m of limestone with 222 peritidal cycles overlain by 273 m of subtidal, non‐cyclic and weakly cyclic limestone
Non‐uniform rates of production, transport and distribution of sediments, superposed on stratigraphic sequences driven by eustasy, probably contributed to the complex cycle patterns recorded in the Dachstein
Summary
Shallow‐water platform carbonates of all ages commonly show repetitive, cyclic patterns. Mechanisms proposed for cycle generation include (a) intrinsic or autocyclic processes and (b) externally imposed or allocyclic processes (Beerbower, 1964), namely eustatic sea‐level changes and tectonic pulses (cf Benedictis, Bosence, & Waltham, 2007; Bosence et al, 2009; Cisne, 1986; Fischer, 1964; Goldhammer, Dunn, & Hardie, 1987). The Dachstein Limestone of the Northern Calcareous Alps may be considered the birthplace of cyclostratigraphy through the seminal work of Sander (1936), he discussed ‘rhythms’ and never used the current term, ‘cycle’. His student, Schwarzacher (1948, 1954), recognized five cycles in a megacycle and postulated orbital control. His student, Schwarzacher (1948, 1954), recognized five cycles in a megacycle and postulated orbital control. Fischer (1964) masterfully elucidated the sedimentary processes forming the Dachstein cycles and speculated about orbital control on cycle development, in particular the obliquity cycle of approximately 41 kyr
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