DETAILED THIN-BEDDED FACIES ANALYSIS OF THE UPPER MANCOS SHALE, NEW MEXICO

Abstract

Our understanding of fine-grained sediment regarding the processes in which they are transported and deposited is rapidly evolving. However, developing a depositional model and characterizing the vertical variability within mud-dominated deposits has seldom been done. A 103m Upper Mancos Shale core retrieved from the San Juan Basin, New Mexico was analysed with detailed thin-bedded facies analysis to observe vertical variability in lithology, sedimentary structures, bioturbation intensity, and depositional processes. Lithological variation suggests there are 3 full sequences, 9 system tracts, and 92 parasequences. Facies observed revealed multiple facies successions indicating depositional processes including ignitive turbidite, storms (tempestite), wave enhanced sediment gravity flows (WESGFs), tidal, biogenic reworking, fluid mud, suspension settling, and general bedload transport. Relationships between lithology, bioturbation intensity, sedimentary structures and depositional processes were observed to be interrelated in that energetic processes (i.e., storms, ignitive turbidite) were associated with coarser deposits and low bioturbation intensity; whereas lower energy processes (i.e., biogenic reworking, suspension settling, WESGFs) were associated with finer deposits and relatively higher bioturbation intensities. Furthermore, lithological variability integrated with depositional models indicated temporal changes in environment of deposition across shelf.