SEQUENCE STRATIGRAPHY OF A REGIONAL–SCALE TIDAL EMBAYMENT, UPPER JURASSIC CURTIS AND SUMMERVILLE FORMATIONS, CENTRAL UTAH

The Upper Jurassic (Oxfordian) Curtis and Summerville sequence begins with pervasive glauconitic, lithic, ebb-oriented ripple-bundle conglomerate deposits filling valleys and arroyos cut into the underlying Entrada Formation during marine reworking of a regional sequence bounding unconformity (J3). Valley fill deposits are overlain by pebbly lag deposits and organic-rich mudstones containing ammonite and crinoid fossils. Lower mudstones are overlain by trough cross-stratified and ripple cross-laminated sandstones and heterolithic units. Heterolithic beds contain vertically accreted sandstone/mudstone laminations, as well as wavy, lenticular and flaser bedding indicating tidal origin including both time-velocity asymmetry and spring-neap succession. In places these deposits display clinoform geometries that have ~25 m of topographic relief and possibly represent tidal sand-wave shoals formed at the mouth of the embayment. Alternatively, clinoform beds may have been deposited during progradation of a tidal-dominated shoreface. Clinoform beds are overlain by trough cross-, and horizontally-stratified sandstones that in places include channel-form geometries. The sand-dominated middle Curtis Fm. is vertically transitional with mud-dominated tidal flat and sabkah facies of the upper Curtis and Summerville formations. The interval is capped by a sequence-bounding unconformity with the overlying Oxfordian-Kimmeridgian Morrison Formation. The Curtis-Summerville interval represents transgressive/highstand systems tract deposition above the J3 unconformity. Valley fill sandstones, organic mudstones, and sandstone/mudstone clinoform beds represent transgressive systems tract deposits laid down during flooding of a broad NE-SW trending marine embayment. Highstand systems tract deposits are represented by the sandy tidal flat/tidal channel deposits of the middle-Curtis Fm. and muddy tidal flat/sabkah deposits of the upper Curtis and Summerville formations deposited. Alternatively, the entire Curtis-Summerville interval above the lower valley fill deposits represent highstand systems tract deposits. Comparison of observed lithofacies assemblages with both modern and ancient tidal depositional sequences may help resolve which of these interpretations is more plausible.