PNNL

Abstract

Siliciclastic strata of the Colorado Plateau attracted attention for their striking red, green, bleached,13 and variegated colors that potentially record both early depositional and later diagenetic events. We14 focus specifically on the proximal-most strata of the Paradox Basin, from their onlap contact with15 Precambrian basement of the Uncompahgre Plateau, to the younger Cutler strata exposed within 1016 km of the Uncompahgre Plateau. These strata preserve complex geology associated with buried17 paleorelief and sediment-related permeability variations. Strata exposed within ~1.5 km of the onlap18 contact exhibit a pervasive drab color in contrast to the generally red colors that predominate farther19 from this front. In-between, strata commonly host variegated red/green/bleached intercalations.20 Mineralogical, geochemical, and textural investigations demonstrate that water-rock interaction21 charged the rocks with Fe(II) that persists primarily in the clay mineral/ phyllosilicate fraction. Color22 variations reflect grain-size differences, allowing reducing fluids from regional fault and23 basement/fill contacts to permeate coarser-grained Cutler sediments. Hematite and chlorite occur in24 both red and green sediments but are absent in the bleached sediments. Pervasive hematite in both red25 and green layers suggests that sediments were hematite-rich before later alteration. Chlorite and26 smectite are elevated in green samples and inversely correlated with biotite. Green coloration is27 generally associated with (1) coarser grain sizes, (2) spatial association with basement contacts, (3)28 elevated smectite and/or chlorite, (4) less total Fe but greater Fe(II)/Fe(III) primarily in the29 phyllosilicate fraction, and (5) uranium enrichment. Abundant mixed layer and swelling clays such as30 smectite, illite-smectite, and chlorite-smectite (including tosudite) dominate the mineralogy of the31 clay fraction. Results are consistent with other studies demonstrating fault-associated fluid alteration32 in the Paradox Basin region. However, the pervasive greening was not observed in many of these33 studies and appears to reflect the unique aspects of the paleovalley system and the importance of34 biotite alteration to Fe(II)-bearing phyllosilicates. Further investigations will be required to unravel35 the specific signatures of parent material, climate, sedimentation, diagenesis, igneous activity,36 hydrocarbon evolution, erosion, and changing regional hydrogeochemical gradients on sediment37 coloration and the potential for paleovalley uranium accumulations along the Uncompahgre Uplift.