Facies control on sandstone composition (and influence of statistical methods on interpretations) in the John Henry Member, Straight Cliffs Formation, Southern Utah, USA

a r t i c l e i n f o The Upper Cretaceous John Henry Member of the Straight Cliffs Formation preserves regressive shoreface and channel facies, and transgressive lagoonal tidal inlet facies that suggest distinct modal sandstone compositions. Detrital modes from six sandstone facies (upper shoreface, lower shoreface, deflected mouth bars, fluvial channels, tidal inlets and washover fans), and their spatial and temporal variations, provide additional data regarding the depositional environments and setting of the John Henry Member. Three statistical methods were utilized: univariate standard deviation and confidence interval and multivariate logratio transformations to assess the compositional difference between facies using estimation of means. Both univariate statistical methods have some inherent problems. Standard deviation does not account for sample size, while the confidence interval method incorporates a t-test to account for only some of the uncertainty of small sample size. The results of the univariate methods differ slightly and both indicate statistically distinct sandstone compositions based on facies. Multivariate statistics are much more robust and suggest similar trends, yet display different compositional means and have inconclusive confidence fields as they incorporate all of the uncertainty associated with small sampling sizes. In general, sandstone compositions become more quartz-rich and compositionally mature as sediment is transported from proximal fluvial environments to distal upper shoreface environments. This trend reflects the degree of preferential reworking or winnowing of unstable grains prior to lithification. More complicated relationships are observed as facies distributions shift through space and time. Sandstone compositions support a deflected wave-dominated deltaic interpretation for the John Henry Member in this area. Sandstone compositions become more lithic-rich from north to south, corresponding to the facies distribution of upper shoreface and deflected mouth bars, respectively. Mouth bars occur at the intersection of the fluvial channel and the coastline where sand is deflected via longshore drift to the southern portion of the field area. Upper shoreface facies located further away from mouth bar facies are heavily reworked by wave processes and contain more mature quartz-rich modal compositions. Additionally, high feldspathic and lithic concentrations in tidal inlet facies suggest that this facies was sourced closer to mouth bars rather than updrift upper shoreface sediments. A shift in sandstone composition occurs at the end of the second transgressive– regressive cycle of the John Henry Member throughout the Rogers Canyon area. This compositional change is concurrent with a relatively large basinward shift in facies, which suggests that …