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Exposed marine sediments of the Imperial Formation in the northeastern margin of the Laguna Salada area, Baja California, provide new insights on the biostratigraphy and paleogeography of the Mio-Pliocene marine incursion in the northern Gulf of California.

The Imperial Formation in the Laguna Salada area includes three informal members based on their lithological and fossiliferous characteristics. A basal (?) conglomerate and breccia, locally up to 70 m thick, overlie the basement rocks in low-angle fault contact. They are interpreted as subaerial to shallow-water gravity and sheet-flow deposits. A middle mudstone member up to 100 m thick is tectonically unrooted and may be laterally equivalent to, or older than, the conglomerate and breccia unit. The mudstone member unconformably underlies or is in fault contact with the upper member, a muddy sandstone unit ~ 140 m thick. The Imperial Formation unconformably underlies ~ 170 m of deltaic sandstone-siltstone of the Palm Spring Formation, and up to 350 m of alluvial fanglomerate.

Microfossil assemblages in the Imperial Formation indicate shallower water depths upsection. Shelf/slope break at a depth that might have exceeded 150 m, is recorded in the mudstone facies. Upsection, the microfossil assemblage and the transition to medium sized sands and interbedded silts indicates a mixed inner shelf and lagoonal facies. No confident age based on planktonic foraminifera, calcareous nannoplankton or palynomorphs was determinable. Up to 90% of the planktonic foraminifera are reworked forms, and the presence of Cretaceous through Tertiary microfossils strongly suggest that the sediments were largely supplied to the basin by the ancient Colorado River delta. In spite of the time transgressive nature of the benthic foraminifera, an age not older than the Pliocene is suggested for the Imperial Formation in the Laguna Salada basin.

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