Depositional History and Neotectonics in Great Salt Lake, Utah, from High-Resolution Seismic Stratigraphy

High-resolution seismic-reflection data from Great Salt Lake show that the basinal sediment sequence is cut by numerous faults with N–S and NE–SW orientations. This faulting shows evidence of varied timing and relative offsets, but includes at least three events totaling about 12 m following the Bonneville phase of the lake (since about 13.5 ka). Several faults displace the uppermost sediments and the lake floor. Bioherm structures are present above some faults, which suggests that the faults served as conduits for sublacustrine discharge of fresh water. A shallow, fault-controlled ridge between Carrington Island and Promontory Point, underlain by a well-cemented pavement, separates the main lake into two basins. The pavement appears to be early Holocene in age and younger sediments lap onto it. Onlap–offlap relationships, reflection truncations, and morphology of the lake floor indicate a low lake, well below the present level, during the early Holocene, during which most of the basin was probably a playa. This low stand is represented by irregular reflections in seismic profiles from the deepest part of the basin. Other prominent reflectors in the profiles are correlated with lithologic changes in sediment cores related to the end of the Bonneville stage of the lake, a thick mirabilite layer in the northern basin, and the Mazama tephra. Reflections below those penetrated by sediment cores document earlier lacustrine cycles

Compilation of detailed examination of Mn deposits from the North tip of the Baja peninsula prior to DSDP Leg 64

he three-site Leg 64 Deep Sea Drilling transect at the tip of the Peninsula of Baja California straddled the transition from continental to oceanic crust. The outer site, 474, penetrated mainly mud turbidites and bottomed in "middle" Pliocene oceanic crust about 3 m.y. old. Two sites on the lower continental slope penetrated hemipelagic muddy sediments, a thin section of low-oxygen, phosphoritic, and glauconitic sediments, and a metamorphic cobble conglomerate; one of the sites, 476, bottomed in deeply weathered granite. The oldest marine sediments at this site are early Pliocene, about 4.5 m.y. old. Depth indicators in these holes suggest that all sites were in almost 1000 meters of water by the time oceanic crust was first generated and sea-floor spreading began. Block faulting, subsidence, and deposition of marine sediments on continental crust had preceded the start of sea-floor spreading. Close examination of lineated magnetic anomalies demonstrates that the transition from continental to oceanic crust in this region is diachronous, as early as 4.9 m.y. in some places, but as young as 3.2 m.y. along the line of the transect. We propose a geological history scenario which involves termination of subduction along the western margin of Baja California at 12.5 Ma, a period of transform motion between the Pacific and North American plates along the Tosco Abreojos Transform Fault zone along the west side of Baja California, and a jump of the Pacific-North American plate edge to the alignment of the Gulf at 5.5 Ma. Between 5.5 Ma and about 3.2 Ma, separation of the blocks occurred locally by sea-floor spreading, but elsewhere by "diffuse extension", largely involving listric normal faulting and thinning of the continental crust, accompanied by subsidence and marine inundation. Thus, the plate edge system in the mouth and southern part of the Gulf evolved as early as 5.5 Ma, but the transition from rifting to drifting was diachronous, starting only 3.2 Ma along the line of the transect