ANATOMY AND ORIGIN OF TOPLAP IN A MIXED CARBONATE-ELASTIC SYSTEM, 7 RIVERS FORMATION (PERMIAN, GUADALUPIAN), GUADALUPE MOUNTAINS, NEW-MEXICO, USA

The Seven Rivers Formation exposed in Slaughter Canyon, Guadalupe Moun tains, New Mexico, reveals complex relations between long- and short-t erm relative changes in sea-level, shelf configuration and sedimentati on, which interacted to create a distinct toplap geometry. At least fi ve sandstones diverge basinward from a prominent boundary unit marking the surface of toplap at the top of the Seven Rivers Formation and cr eate a series of prograding, shingled clinoforms. The boundary unit is a horizontal, well-sorted, quartz arenite underlain across the shelf by peritidal carbonate or by other merging sandstones. Preserved palae otopography is indicated by facies changes downdip and the presence of horizontal geopetal indicators in inclined beds. Near the boundary un it (updip), merging sandstones contain rare sedimentary structures inc luding evaporite moulds and irregular fenestrae and are bounded above and below by peritidal carbonate with microbial laminae, fenestral fab rics and mudcracks. Laterally (downdip), the sandstone-bounding periti dal carbonate facies pass into subtidal carbonate facies (ooid-peloid- fusulinid-dasyclad-mollusc pack- and grainstone) and interbedded sands tones contain sedimentary structures such as ripple marks and trough t o planar cross-stratification, as well as ooids, fusulinids and other carbonate grains. Toplap is interpreted to have developed by sediment bypass across a subaerially exposed shelf while sedimentation continue d in still-submerged areas downdip from the shelf crest, and hence rep resents depositional toplap. Physical tracing of subaerial exposure su rfaces suggests that the shoreline migrated up and down palaeoslope se veral times. The vertical component of five short-term shoreline migra tions decreased during formation of the toplap geometry. Sea-level ros e to approximately the same position following each fall to create the toplap geometry. This depositional toplap is the stratigraphic result of high-'frequency' relative changes of sea-level that combined to pr oduce the larger-scale geometry. We suggest that changing amplitudes o f relative sea-level may play a significant role in the stratigraphic evolution of platforms and that separating 'short-term' and 'long-term ' relative sea-level may be ambiguous in such instances.