Three-dimensional stratigraphic architecture and evolution of an ancient river-dominated delta, Iles Formation, Book Cliffs, Colorado

Abstract

The architecture and evolution of an ancient river-dominated delta in the eastern Book Cliffs is documented in a three-dimensional analysis of one parasequence in the Iles Formation. The mouth bars of parasequence 3 (PS 3) have three distinct regions, which are, from updip to downdip: proximal, medial, and distal. The proximal mouth bar is characterized by convex-up units of trough cross-stratified sandstone. The medial mouth bar region is characterized by steeply-dipping (ca. 5°) clinoforming units of amalgamated, planar-laminated sand that transfer downdip to the distal mouth bar region, which is characterized by shallowly-dipping (less than 0.5°), heterolithic strata with partial Bouma sequences. This downdip transition in facies is interpreted to reflect a transition in depositional process, from bedload and traction in the proximal mouth bar to sediment gravity flow in the medial and distal mouth bar. The stacking of mouth bars is hierarchical: beds build bedsets, bedsets build stories, and stories build elements. Mouth bars stack in different directions according to hierarchical scale: stories stack basinward whereas elements stack laterally. The stratigraphic architecture of PS 3, which was deposited in a minimum of 15 meters of water, contrasts with that of a shallower-water (ca. 4 meters) delta documented in the Neslen Formation by a companion study. There is significant aggradation in PS 3 of the silt-rich bottomset, which reduces the water depth in which subsequent mouth bars are deposited. The maximum clinoform angle is 5° and mouth bars are predominantly deposited by sediment gravity flow. In contrast, there is little bottomset aggradation in the shallower-water delta. The maximum clinoform angle is 3° and mouth bars are predominantly deposited by bedload and traction. Mouth bar elements stack more compensationally in the shallower-water delta than in PS 3. Finally, this study suggests that bottomset aggradation should be accounted for in numerical stratigraphic models of river-dominated deltas.