Recognition and significance of sharp-based mouth-bar deposits in the Eocene Green River Formation, Uinta Basin, Utah



Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth-bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive-transgressive sequence. Individual sandstone bodies are texturally sharp-based towards mudstone substratum. In proximal parts, the mouth-bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth-bar clinothems. The clinothems pass gradually into greenish-grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex-upward sandstone bars. The mouth-bar deposits are connected to terminal distributary channel deposits. Together, these mouth-bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth-bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.