ELSEVIER MarineGeology 123 (1995) 1-10 MARINE GEOLOGY INTERNATIONAL JOURNAL OF MARINE GEOLOGY, GEOCHEMfSTRY AND GEOP;'fYSICS Modeling of cyclic tidal rhythmites based on a range of diurnal to semidiurnal tidal-station data Allen W. Archer Department of Geology, Kansas State University, Manhattan, KS 66506-3201, USA Received 12 January 1994; revision accepted 31 October 1994 Abstract Where individual tides deposit appreciable accumulations of sediment, cyclic tidal rhythmites can form that exhibit semimonthly tidal cyclicities.A number of modern analogs and ancient counterparts indicate the importance of such features for paleoenvironmental analyses. In this study, predicted, high-low, tidal-station data are used to simulate current speeds, which in turn are used to simulate sedimentation of tidal rhythmites. This modeling has been done to depict the patterns encoded in cyclic tidal rhythmites. In order to delineate the spectrum of different tidal regimes, the simulations are based upon a series of macrotidal, diurnal to semidiurnal tidal systems.Depending upon thresholds of relative tidal-current speed, only portions of semimonthlytidal cycleswill be preserved within cyclictidal rhythmites. 1. Introduction In some tidal deposits, sedimentation is capable of producing variations in thicknesses of successive foresets or laminae. These thickness variations can produce what have been termed "neap-spring cycles" described from inclined foresets (Visser, 1980; Allen, 1981). Vertically accreted lamina also exhibit such cycles in modern settings (Dalrymple and Makino, 1989; Tessier et al., 1989) and ancient examples (Sonett et al., 1989; Williams, 1989; Kvale et al., 1989; Kuecher et al., 1990). These deposits have been generically termed "tidal rhyth- mites" (Williams, 1989), however, this term is problematic because "rhythmite" can refer to either each individual foreset or lamina or to the cycles exhibited by a series of such sedimentation units. In order to reduce this potential confusion, the term "cyclic tidal rhythmite" is used to refer to tidal deposits that exhibit semimonthly (neap- spring) cycles. Previous modeling of cyclic tidal rhythmites 0025-3227/95/$09.50 © 1995ElsevierScience B.V.All rightsreserved SSDI 0025-3227(94)00113-8 (Archer, 1991; Archer et al., 1991) used predicted tidal data, in which tidal heights were used as a proxy for the thickness of tidally produced lamin- ations. In this paper, more realistic modeling is based upon the use of simulated current speeds to generate rhythmite patterns. Modern analogs for intertidal rhythmites have been described from the Bay of Fundy in Nova Scotia, Canada (Dalrymple and Makino, 1989; Dalrymple et al., 1991) and the Bay of Mont Saint Michel in northwestern France (Tessier, 1993; Tessier et al., 1989). These areas exhibit some of the highest tidal ranges in the world and spring-tidal ranges approach or exceed 15 m. Within these extremely dynamic, tide-dominated sedimentary environments, there is widespread deposition of laminae that exhibit tidal periodicities. These modern analogs of cyclic tidal rhythmites form in estuarine settings where high levels of turbidity result in very high rates of sediment accumulation during individual tidal cycles. The intent herein is to simulate sedimentation