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Crowded tubular tidalites in Miocene shelf sandstones of southern Iberia
- Rodríguez-Tovar, Francisco J., Mayoral, Eduardo, Santos, Ana, Dorador, Javier, Wetzel, Andreas
- Palaeogeography, palaeoclimatology, palaeoecology 2019 v.521 pp. 1-9
- Miocene epoch, burrows, periodicity, sandstone, tides, Iberian Peninsula, Portugal
- The passive and active fill of burrows potentially stores information about sedimentary processes that are otherwise not preserved in the rock record. In recent years, abandoned passively-filled vertical burrows were introduced as “tubular tidalites” when their infilling displays rhythmic lamination reflecting a tidal signature. In the shallow-marine Miocene sandstones exposed at Oura (southern Portugal), 36 tubular tidalites occur in a 1.5 m-thick interval. Their high abundance is likely a consequence of both an environment favourable for the production of open burrows in a tidal setting, and post-depositional conditions facilitating the preservation of the tubular tidalites. Besides vertical tubes, 13 horizontal burrows preserve a tidal signature indicating draught-fill processes. All specimens belong to Thalassinoides and, for the first time, to Gyrolithes. The rhythmic infill of two well-preserved specimens shows two significant features: (1) The thickness pattern allows for differentiation into groups having 7 couplets (consisting of a dark and a light lamina) or multiples thereof, and (2) the thickness patterns of both, consecutive couplets as well as dark and light laminae match sine curves. Both patterns indicate a diurnal tidal cyclicity. The tidalites record up to four spring-tide and three neap-tide cycles. In addition to the neap-spring cycles, a long-period lunar fortnightly tide regime can be envisaged. The tubular tidalites imply diurnal tides during the Miocene in contrast to the Recent semidiurnal tides affecting southern Portugal.