Abstract

The characteristic cyclicities displayed by clastic tidal deposits are unique criteria for recognizing tidal dominance in shallow marine clastic deposits.

Cyclicities detected in megaripple crossbedding structures that formed in tide-dominated environments include: 1) the bundling of sandy foresets bounded by mud couplets and/or a mud drape and a reactivation surface indicating floodebb cycles; 2) the thick-thin alternation of successive bundles reflecting the diurnal tidal inequality; 3) the lateral bundle thickness variation and bottomset thickness variation related to semi-lunar cycles of neap-spring tides; and 4) the variation in spring bundle thickness related to the lunar cycles of high-spring/low-spring tides. Reactivation surfaces can be related to the flood-ebb cycle (bidirectional reactivation surface) as well as the neap-spring tidal variation (unidirectional reactivation surface with a regular wavy erosional pattern).

Cyclicities observed in delicate, vertically-stacked, thinly-laminated tidal rhythmites include: 1) the sand/mud couplets related to peak flow/stillstand cycles, which can be correlated to flood-ebb cycles if evidence for other tidal cyclicities can also be found; 2) the vertical couplet thickness variation related to changes in sand transport and mud concentration during diurnal and neap-spring tidal cycles; 3) the variation in the thickness of spring couplets indicating the lunar cycles of high- and low-spring tides; and 4) the vertical thickness variation of rhythmites, reflecting longer cycles of tidal variations or seasonal changes in sediment concentration.

These unique criteria have been established based on comparative studies of modern tidal processes and associated preserved structures and sequences of tidal deposits. Numerous applications of these diagnostic criteria have shown their usefulness in sedimentary facies interpretation of outcrop and subsurface sequences. A clear recognition of tide-dominated sequences can be of importance to sequence stratigraphy and basin analysis (e.g., spatial facies development versus sea level fluctuations).