CONSTRAINTS ON THE FORMATION OF PLIOCENE HUMMOCKY CROSS-STRATIFICATION IN CALABRIA (SOUTHERN ITALY) FROM CONSIDERATION OF HYDRAULIC AND DISPERSIVE EQUIVALENCE, GRAIN-FLOW THEORY, AND SUSPENDED-LOAD FALLOUT RATE

Hummocky cross-stratification (HCS) occurs in two Pliocene progradational nearshore-marine to fluvial sequences in the town of Guardavalle, Calabria (southern Italy). The HCS is notable because of its shallow depth of deposition (~ 2-5 m) and its coarse grain size (up to coarse sand). Individual units of HCS consist of, in ascending order, three subfacies which are characterized by distinctive internal structure: a massive subfaeies; a centimeter-scale laminated (meso-laminated) subfacies; and a millimeter-scale laminated (micro-laminated) subfacies. The massive subfacies consists of norrnally graded, coarse to fine sand (9 = 1.98-1.85), in beds a few crn to ~25 cm thick. These beds have fiat to broadly undulatory bases and hummocky-swaley tops. The meso-laminated subfacies consists of sets of 0.8- to 1-cm-thick laminae of normally graded, medium to fine sand (9 = 2.3) that drape the underlying massive subfacies, thicken gradually toward swales, and then flatten upward. The micro-laminated subfacies comprises sets of several, 2- to 8-mm-thick, planar to slightly undulatory laminae that are characterized by mica-rich and mica-poor zones. Individual micro-laminae are inversely graded, from silt and very fine sand at their bases to fine to medium sand at their tops (ff = 2.84-2.95). The upper few tenths of individual laminae are highly enriched (up to 56%) in biotite grains, most of which are aligned parallel to the lamination. The most likely mode of deposition of the massive and meso-laminated subfacies was rapid settling from suspension from increasingly unsteady flows. The bulk of the sediment was probably imported to the depositional site by unidirectional flows during major storms. Hummocky-swaley laminae did not develop during deposition of the massive subfacies because of the high rate of suspended-load fallout (~), which probably was controlled by the relatively coarse grain size. The grains composing the micro-laminae are in dispersive, not hydraulic, equivalence. The thickness/grain-size relationships within micro-laminae satisfy predictions of grain-flow theory and indicate that the laminae were produced by collapse of traction carpets under the influence of currents on the order of 80-150 cm/s. Deposition of the Guardavalle HCS took place in four stages: 1) initial plane bed; 2) growth of hummocky bedforms while tk was very high, probably under the influence of combined orbital and unidirectional currents; 3) draping of the hummocky bedforms by meso-laminae during a period of moderately high tk, high orbital velocity, but lower unidirectional-current velocity; and 4) reworking of the seafloor by powerful orbital currents under conditions of relatively low ~ and minor unidirectional currents. Preservation of the Guardavalle HCS in a very shallow-marine environment was promoted by the mierotidal, storm-wave-dominated character of the Pliocene Ionian coast of Calabria.