TUBULAR TIDALITES: A BIOGENIC SEDIMENTARY STRUCTURE INDICATIVE OF TIDALLY INFLUENCED SEDIMENTATION

The most conclusive evidence of tidal sedimentation consists of deposits that exhibit evidence of rhythmic sedimentation attributable to tidal cyclicity. Until now such deposits have been limited to physical sedimentary structures. Tubular tidalites, defined herein, are combined biogenic-physical sedimentary structures that provide compelling evidence for tidal mediation in sedimentary successions. These structures consist of rhythmically bedded alternating layers of fine-grained and coarse-grained laminae deposited within open framework burrows such as Arenicolites, Ophiomorpha, Palaeophycus, Psilonichnus, and Thalassinoides. Importantly, the open framework burrows represent natural sediment traps that potentially preserve depositional cycles that might be missing due to high levels of bioturbation in settings such as intertidal flats and lagoons. The interpretation of the burrow infills as being tidally modulated is tested and supported in three ways: using exponential smoothing of thickness data on burrow-infilling laminae to identify rhythmic trends in laminae thicknesses, visually fitting sinusoidal curves to the raw thickness distributions, and using Fast Fourier Transform analysis. Considering the morphology of tubular tidalites presented, the parameters that promote the occurrence of tubular tidalites include an open aperture, bimodal sediment availability, tidal influence in sedimentation, and low overall sedimentation rates. Tubular tidalites have been identified from tidally influenced successions in Mesozoic, Cenozoic, and modern deposits and provide evidence of tidal influence in sedimentary successions where other indicators may be ambiguous.