Crowded tubular tidalites in Miocene shelf sandstones of southern Iberia

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.