SEQUENCE STRATIGRAPHY OF THE MIDDLE CAMBRIAN MARJUM FORMATION: RESPONSE OF SEDIMENTARY FACIES AND BIOTA TO SEA LEVEL CHANGES

Recent study on the Marjum Fm provides insight into facies transitions in an embayment existing in southwestern Utah during the Middle Cambrian. Sections of the Marjum Fm and its equivalent have been measured in shallow- (Drum Mountains), intermediate- (Wheeler Amphitheatre), and deep-ramp (Marjum Pass) environments. The Drum Mountains section contains peritidal deposits with some evidence of subaerial exposure. Lithologies at this locality are dominated by calcisiltite and dolomite with some oolitic and fossiliferous packstone, laminated mat to large (3m tall) mounded stromatolite boundstone, and very minor shale. At Wheeler Amphitheatre, intermediate-ramp deposits are largely fossiliferous, oolitic, and oncolitic wacke/packstone with significant shale, calcisiltite, and dolomite. There are also minor limestone-shale rhythmite and small (40cm tall) mounded stromatolite boundstone lithologies. Deep-ramp deposits dominated by shale, calcisiltite, limestone-shale rhythmite, and mudmounds characterize the section at Marjum Pass. In each location, stacking patterns and facies changes occurring up section suggest the Marjum Fm represents an overall shallowing upward sequence (or possibly sequences). Rates and magnitudes of sea level changes have been determined by using lithology, TOC, and body and trace fossils. Polymerid and agnostid trilobites, along with other marine fauna, aid in constraining time of deposition. Facies patterns, lithology, geochemistry (TOC), and fossil content delineate bounding surfaces that separate (and aid in the identification of) individual parasequences and systems tracts. These sequence stratigraphic features are correlated across the embayment and used to identify responses of different sedimentary environments to fluctuations in sea level. Parasequences are well expressed in mid- to deep-ramp settings, but less clearly defined, perhaps due to diagenesis, in peritidal settings. Research on the Marjum Fm is valuable because it allows documentation of basin evolution within an isolated embayment cutting across a passive margin. Understanding the controls on biostratigraphy, facies distribution, and migration of potential source rocks over time within the Marjum Fm will aid in developing improved hydrocarbon exploration models.