A MID-PROTEROZOIC BROAD, SHALLOW LAKE: HELENA FORMATION, BELT SUPERGROUP, WESTERN MONTANA AND NORTHERN IDAHO

Precambrian lake deposits are difficult to differentiate from marine deposits chiefly because Precambrian rocks lack fossils of multicellular biotas. Lake processes are in many ways similar to marine processes, and their records may also appear similar. The 1.45 Ga Helena Formation of the Mid-Proterozoic Belt Supergroup was deposited in an intracratonic block-fault basin and is inferred to be lacustrine on the basis of its styles of its 1-10 m-thick sedimentary cycles. The Helena lake was at least 500 km N-S and 300 km E-W. Based on description and correlation of more than 30 measured sections, vertical and lateral facies patterns were identified and interpreted. Layers more than 3 cm thick of hummocky very fine sand layers capped by dark mud occupy the lake center. They thin and fine shoreward to undulating silt-to-mud graded layers 3.0 to 0.3 cm thick. These continue to thin and fine to graded layers less than 0.3 mm thick across the lake margins. The lake lacked a clinoform and fondoform, and its floor was exceedingly flat, shallow, and everywhere above storm wave base. Mud and very fine sand were continually reworked by storms in the center of the lake, and mud was driven to the lake margins.

Helena cycle bases record exposure around the edges of the lake, but not below base level in the lake center. Lower half-cycles record lake expansion and development of a siliciclastic highstand facies tract, with with water undersaturated with respect to Ca, Mg, CO3. Upper half-cycles record lake contraction, with only slight regression of the siliciclastic facies tract. However, lake waters became supersaturated with respect to Ca, Mg, CO3, and dolomite mixed with siliciclastic facies accumulated across most of the basin.