CHALLENGING TRADITIONAL FLUVIAL FACIES MODELS - NEAR-SURFACE DIAGENESIS IN LATE JURASSIC TO EARLY CRETACEOUS FLUVIAL DEPOSITS OF EASTERN UTAH, USA

An increasing number of researcher question established facies models and their application to predict lithofacies heterogeneities. In fluvial facies models flood basins experience subordinate influence from active depositional processes and are commonly displayed as being depleted of facies details. The floodplain is considered the realm of pedogenesis which is deemed to have no influence on channel belt lithofacies.

Near Green River, Utah, deposits of uppermost Morrison Fm. and lower Cedar Mountain Fm. reveal parts of the internal drainage of the North American Cordillera. Remarkably, depositional architecture largely escaped erosion due to intensive silica cementation. Cementation preferentially affected fluvial sandstones and is less pervasive in floodplain paleosols. In channel facies, cemented domains of extremely reduced porosity/ permeability define zones of significant vertical and lateral extent.

Observation of opaline silica lining pedotubules and cracks in calcite nodules indicates that cementation was initiated during or shortly after deposition of fluvial facies. Microfabrics such as peds, cutans, silica mineraloid crusts and cryptocrystalline silica matrices confirm a pedogenic origin of the silica cements. In absence of soil features, floating-fabrics and circumgranular silica rinds, contrasting packing indices and cross-cutting of silica fronts with depositional structures relate to shallow groundwater silicification. Silica was either provided by cannibalization of sandstone grains likely when groundwater alkalinity was high. Alternatively, silica was released during breakdown of clay minerals in soil environments. The complex paragenetic relationships between calcite and silica phases reveal that chemical conditions frequently fluctuated in the system.

Near-surface diagenetic processes commonly cross-cut the boundaries of depositional lithofacies and significantly modify fluvial reservoir lithofacies. Their distribution contrasts with traditional heterogeneity models, which predict spatial variations and continuity of facies merely on the basis of depositional processes. This paper aims to incorporate the effects of near-surface early diagenesis to improve the applicability of traditional, deposition-controlled fluvial facies models.