Paleoenvironments of Forbes Formation: ABSTRACT

Abstract

The Upper Cretaceous Forbes Formation of California's Great Valley is a sequence of mudstones, shales, and sandstones, interpreted as lower slope-fan fringe to midfan facies deposited in a forearc basin. It contains two different foraminiferal faunas: one is largely agglutinated; the other, restricted to the lowermost Dobbins Shale Member, is more diverse. These faunas are the result of turbiditic sedimentation and differential preservation. The Dobbins--a limy, concretionary mudstone--represents low net accumulation rates above the carbonate compensation depth, but below the foraminiferal lysocline. Oxygen levels were low, but not continuously anaerobic. Sediment texture is nonlaminated, but the preservation of calcareous foraminifera suggests bioturbation was intermittent and not extensive. An increase in sedimentation rates occurs in the middle Forbes, with an increase in the number and thickness of sand beds. The lack of calcareous species is largely the result of episodic sedimentation and increased bioturbation. In the upper member, the number of sand beds and the accumulation rate decrease. Paleobathymetric estimates, based on agglutinated foraminifera, suggest an upper abyssal-lower bathyal depth throughout the formation, with an indication of minor shallowing upsection. The Santonian/Campanian boundary occurs at the top of the Dobbins shale, as marked by the first appearance of the calcareous nannofossil, Broinsonia parca. The Forbes Formation is included in the Globotruncana arca planktonic foraminifera zone. End_of_Article - Last_Page 681------------