ORIGIN AND EVOLUTION OF MEGABRECCIA DEPOSITS ON THE SLOPE OF A CENOMANIAN CARBONATE PLATFORM, SOUTH-CENTRAL PYRENEES, SPAIN

The Sopeira Breccia (Cenomanian) consists of a thick succession of carbonate megabreccia and reworked sediments deposited on the slope of a carbonate platform that was located on the southern margin of the mid-Cretaceous Pyrenean rift basin, which formed between present day France and Spain. These deposits are restricted to a small sub-basin near the town of Sopeira, Spain. Three main facies have been defined, which include undisturbed slope mudstone to wackestone (Facies 1), megabreccia with slope-derived clasts (Facies 2), and megabreccia with slope- and margin-derived clasts (Facies 3). Facies 1 and 2 are interpreted to represent background deposition on an over-steepened slope, whereas Facies 3 represents erosion and redeposition of shallow platform facies.

These facies are arranged into four cycles that, when complete, are composed of Facies 1 and 2 in the lower portion, followed by Facies 3, and a thin cap of Facies 2 containing small clasts. The relative quantity of margin-derived clasts (Facies 3) increases from cycle 1 (at the base) to cycle 4. Cycle 1 contains discontinuous deposits of Facies 3 that appear to be confined to small channels. Facies 3 comprises a larger portion of the cycles 2 through 4, and it occurs in more continuous layers on the lower slope. Cycles 2 to 4 also contain the cap of Facies 2 with small clasts. Cycle four differs in that the top portion is mostly composed of Facies 2. A thick succession of Turonian-aged deeper water deposits occurs above cycle 4.

The cycles are interpreted to reflect changing sea level with the introduction of margin-derived clasts when sea level was low. Cycles 1 to 3 record a lower order fall in relative sea level, whereas cycle 4 and the overlying Turionian deposits record overall deepening. Deposition of the breccias appears to have been controlled by a normal fault that localized the platform margin in the sub-basin near Sopeira. An increase in the amount of margin-derived clasts from cycle 1 to cycle 4 suggests back-stepping erosion along the fault scarp, eventually cutting through the margin and into the platform interior. Elsewhere in the Pyrenean Basin, coeval slopes lack megabreccia and display a ramp-like geometry. The fault in the sub-basin near Sopeira resulted in a steep slope geometry and provided a possible triggering mechanism for the megabreccia.