Pliocene volcanogenic sedimentation along an accommodation zone in northeastern Baja California: The Puertecitos Formation

Accommodation zones on rift systems produce distinctive sedimentary facies and facies architecture. The Puertecitos volcanic province in the northern Gulf Extensional Province comprises an accommodation zone and records marine sedimentation and explosive volcanism during upper Miocene and early Pliocene time. The volcano-sedimentary sequence consists of two ·westward-thinning, wedge-shaped transgressive-regressive marine sequences, each less than 100 m thick, separated by one large pyroclastic flow unit. Northward the lower member is separated from the upper member by an angular unconformity and/or an interval of subaerial erosion. To the southeast, the volcanic units dominate the stratigraphic sections, whereas northward the two marine sequences dominate and contain the distal volcaniclastic facies. We propose the name of Puertecitos Formation to formally name this volcanosedimentary sequence. The sedimentary facies crop out in subparallel narrow belts along the present range front to the east of the volcanic province. The fossil assemblages and the sedimentary facies show that this area was a tide-dominated marine embayment and alluvial plain, deepening to the east. Although the environment is favorable for the formation of bioclastic carbonates, the section is essentially devoid of significant carbonate deposits. Instead, the basin was filled with epiclastic and pyroclastic material, and individual volcanic units occur as both subaerial and submarine facies and provide excellent chronostratigraphic markers. Isostatic sea-level changes and volcaniclastic deposits had a major impact on the facies distribution. Sediment accumulation rates were low. Pre- and postsedimentary deformation was produced by an evenly distributed array of normal faults with small individual offset. This fault pattern produced local facies variations and a distinct facies architecture that contrasts with sedimentary facies associated with basins on rift segments to the north and elsewhere in the Gulf Extensional Province, where rapid subsidence and coarse-grained deposits are associated with large vertical separation on basin-bounding faults.