The Transition Between Sheet-Like Lobe and Basin-Plain Turbidites in the Hecho Basin (South-Central Pyrenees, Spain)

Abstract

Genetic facies analysis based on bed-by-bed correlations from sheet-like lobes to the basin plain in some Hecho turbidite systems demonstrates that at least 50% of the flows building the sheet-like lobes kept moving downcurrent to the closely related basin plain and underwent flow transformations, interpreted to have resulted from interaction with topography at the basin margin(s), that gave rise to specific facies. These facies form a new facies tract that replaces the fine-grained group of turbidite facies (very fine sand to mud) and characterizes the basin-plain beds. Beds in the sheet-like lobes evolve downcurrent in a way that is predictable by the existing turbidite facies tract models, whereas 36% of basin-plain beds, which account for 78% of the basin-plain volume, do not. The latter have deposits from high-density turbidity currents at their bases and typically complete basin-floor coverage. The new facies tract developed when the flows obliquely encountered the southern foreland-margin ramp. At the ramp, the lower, sand-laden and high-density part of the larger flows was deflected, evolving downcurrent along the ramp trend. The upper part of the flow, more dilute and thicker, was reflected from the foreland margin as a train of declining undular bores (moving hydraulic jumps). Subsequent reflections generated against the flanking margins in the closed basin led to ponding, which resulted in an overall sheet-like stacking pattern across sheet-like lobes and the basin plain and is the diagnostic feature of the distal element in the lower, sand-rich stages of the turbidite systems. Calcilutites on top of beds, interpreted until now as hemipelagites, show field evidence of having a turbiditic origin (hemiturbidites), thus forming the facies capping the new facies tract.