FRONTAL AND LATERAL SUBMARINE LOBE FRINGES: COMPARING SEDIMENTARY FACIES, ARCHITECTURE AND FLOW PROCESSES

摘要

Submarine lobe-fringe deposits form heterolithic successions that may include a high proportion of hybrid beds. The identification of lobe-fringe successions aids interpretation of paleogeographic setting and the degree of basin confinement. Here, for the first time, the sedimentological and architectural differences between frontal and lateral lobe-fringe deposits are investigated. Extensive outcrop and core data from Fan 4, Skoorsteenberg Formation, Karoo Basin, South Africa, allow the rates and style of facies changes from axis to fringe settings of lobes and lobe complexes in both down-dip (frontal) and across-strike (lateral) directions to be tightly constrained over an 800 km(2) study area. Fan 4 comprises three sand-prone divisions that form compensationally stacked lobe complexes, separated by thick packages of thin-bedded siltstone and sandstone intercalated with (muddy) siltstone, interpreted as the fringes of lobe complexes. Lobe-fringe facies associations comprise: i) thick-bedded structureless or planar-laminated sandstones that pinch and swell, and are associated with underlying debrites; ii) argillaceous and mudclast-rich hybrid beds; and iii) current-ripple-laminated sandstones and siltstones. Typically, frontal fringes contain high proportions of hybrid beds and transition from thick-bedded sandstones over length scales of 1 to 2 km. In contrast, lateral fringe deposits tend to comprise current-ripple-laminated sandstones that transition to thick-bedded sandstones in the lobe axis over several kilometers. Variability of primary flow processes are interpreted to control the documented differences in facies association. Preferential deposition of hybrid beds in frontal fringe positions is related to the dominantly downstream momentum of the high-density core of the flow. In contrast, the ripple-laminated thin beds in lateral fringe positions are interpreted to be deposited by more dilute low-density (parts of the) flows. The development of recognition criteria to distinguish between frontal and lateral lobe-fringe successions is critical to improving paleogeographic reconstructions of submarine fans at outcrop and in the subsurface, and will help to reduce uncertainty during hydrocarbon field appraisal and development.