Tectonically-Controlled Submarine Canyon Initiation, Fill and Abandonment Constrained by Detrital Zircon Geochronology: Cretaceous Punta Baja Fm., Baja California, Mexico
Submarine canyons incise the Earth’s continental shelves, and act as conduits for large volumes of sediment transferred from terrestrial to deep-marine basins, but as long-lived bypass-dominated systems their fills are considered complicated and unprospective. Canyon initiation and growth is poorly constrained as their early evolution is obliterated by subsequent incision. Here, we examine a coarse-grained submarine canyon-fill, the Punta Baja Fm., which formed in a tectonically active setting within the Peninsular Ranges forearc basin complex. Early strike-slip faulting is interpreted to have imparted paleocoast-parallel structural lineaments in the bedrock, the fluvial Bocana Roja Fm., which were exploited by coarse-grained sediment gravity flows. In a strike section along the canyon base, there is evidence for progressive deepening, from margin to axis, of incisions associated with faults in the bedrock. The canyon-fill is ~120 m thick and up to 1.2 km wide, and comprises conglomeratic channel bodies with sand-rich margins that pass into thin-bedded successions interpreted as internal levees. The channel-fills are associated with, and overlie, mass transport deposits containing large blocks (meters to tens of meters) of bedrock and canyon-fill deposits. Active tectonism is demonstrated by faulting patterns and basinward rotation of the bedrock, followed by incision of the canyon. The canyon fill, whilst suggestive of significant sediment bypass, was also strongly aggradational suggesting high sediment supply rates and increasing accommodation. The canyon was buried, rotated basinward, eroded and onlapped unconformably by the shallow-marine El Gallo Fm. Preliminary sandstone maximum depositional ages (MDAs) from detrital zircons date the fluvial Bocana Roja Fm. at 95.3±1.4 Ma. The Punta Baja Fm. canyon-fill is dated at 89.6±1.5 Ma to 87.8±1.5 Ma, and the El Gallo Fm. at 88.6±1.5 Ma, which is statistically indistinguishable from the canyon-fill ages suggesting a very close age relationship. The structural, sedimentological and geochronological evidence suggest that the structurally controlled deep-marine canyon developed and filled relatively rapidly, preserving thick successions of overbank deposits within the canyon. The shelf was then rotated basin-ward, uplifted, eroded and a shallow marine shelf was established over the top at a time-scale below the resolution of the detrital zircon chronology. This study provides new insights into the potentially relatively short lifespan of major submarine canyons in tectonically active settings, and the coarse-grained character of their fill.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019