Print this pageGuidelines for Application of Stacking Patterns Techniques in Platform Carbonates From Ice-House to Green-House Periods of Earth History
LEHRMANN, D. J., and R. K. GOLDHAMMER,
Recent debate as to whether cyclic shallow-marine carbonate strata preserve a record dominated by random (episodic) or deterministic (periodic) processes points to the likelihood that a spectrum of stratigraphic behaviors exists that may be partly explained by secular variations in the strength of allocyclic forcing and the degree to which sediment fill faithfully records accommodation space. Stacking-patterns techniques would therefore need to be adjusted accordingly in order to construct more accurate chronostratigraphic zonations. To test these ideas we have analyzed high-quality stratigraphic data from over 50 depositional systems with ages ranging through the Phanerozoic. Levels of order were evaluated for thickness and facies succession and lateral continuity using a variety of statistical analyses, comparison of normalized graphic sections, and qualitative survey of continuities.
High-amplitude sea-level fluctuations of ice-house periods generate stacks of high-frequency sequences capped with exposure surfaces. Strong allocyclic forcing-potential results in high continuity and relatively ordered vertical facies successions with non-Waltherian offsets at sequence boundaries. Incomplete recording of accommodation space, however, yields uniform to chaotic vertical thickness partitioning and poorly-defined long-term changes in facies proportions resulting in ambiguity in selection of "3rd order" composite sequence-boundaries in the platform interior. Retrogradational to progradational patterns of platform margin facies best define composite sequences.
Lower amplitude sea-level fluctuations of green-house periods results in greater levels of autocyclicity at the parasequence scale and more faithful recording of accommodation history at the parasequence-set and "3rd order" sequence scale. The superposition of low-amplitude, high-frequency sea-level fluctuations on higher-amplitude fluctuations of lower frequencies sets up the potential for recording a full hierarchy (3,4,5th orders) and possibly a Milankovitch signal given an ideal "tuning" of sedimentation, subsidence, and environmental conditions. Green-house sequences can be confidently identified and correlated based on vertical thickness and facies stacking patterns and are bounded by minimum accommodation zones, defined by maximum regressive facies development and/or thin peritidal- prone parasequences.