Abstract:
A sand provenance study was conducted to test the hypothesis that the middle Miocene Magdalena fan was sourced near the mouth of the proto-Gulf of California and then laterally displaced to the northwest. Detrital modes of ten sandstone turbidite beds cored at Deep Sea Drilling Project (DSDP) Site 471 on the Magdalena fan were petrographically determined and compared to those of seven modern onshore stream and beach sand samples, as well as 31 samples from DSDP Sites 474, 475, 476 and 485 in mouth of the Gulf of California. Mean QFL percentages (22%Q:32%F:46%L) for Magdalena fan sandstones are similar to those of Pliocene sand at Sites 474-476 (19%Q:36%F:44%L) and to Quaternary sand at Site 485 (17%Q:32%F:51%L). These similarities persist in the monomineralic and lithic proportions with the exception that the carbonate-cemented Magdalena sandstones contain slightly less feldspar, owing to some diagenetic replacement by carbonate. The lithic fractions for all are dominated by volcanic clasts. The source of sand at Site 485, located near the East Pacific Rise in the mouth of the Gulf of California, was likely the Mexican mainland near El Dorado, whereas the sand at Sites 474-476 was likely derived from the southernmost tip of Baja California. All of the above QFL means plot within the transitional arc field, a characteristic of sand with an arc-related triple-junction provenance. In contrast, the Magdalena Fan sandstones are compositionally distinct from modern sand collected from onshore streams and beaches to the east of its present location off southwestern Baja California. The modern sands group into three compositional suites, one volcaniclastic and two quartzofeldspathic; the volcaniclastic group (mean 2%Q:23%F:75%L) is associated with outcrops of early to middle Miocene volcaniclastic rocks, whereas the quartzofeldspathic suites are associated with outcrops of either the Mesozoic subduction complex on Santa Margarita Island (mean 47%Q:39%F:14%L) or Eocene sedimentary units (mean 30%Q:54%F:6%L). These data are consistent with a model whereby the Magdalena fan was sourced during the middle Miocene at a triple junction at the mouth of the proto-Gulf of California and later displaced 300 km northward, perhaps along the Tosco-Abreojos fault zone.