PROPOSED TECTONIC PATTERNS AND HISTORY OF THE MIDDLE PROTEROZOIC BELT BASIN DURING DEPOSITION OF THE HELENA AND WALLACE FORMATIONS

Stratigraphic and sedimentologic study of the redefined Helena and Wallace formations may reveal tectonic patterns of Precambrian basement blocks and the tectonic history of these formations. Sedimentary cycles low in the Helena Formation contain mostly undulating couplets in their lower parts and dolomitic undulating couplets, even couplets and microlaminae in their upper parts. They appear to record expansion and contraction of a very shallow "Belt sea" or lake. Higher is a lens of cycles with pinch-and-swell couples and couplets in their lower parts and pinch-and-swell couplets and undulating couplets in their upper parts. They probably record expansions and contractions of a broader Belt sea or lake with more fetch, but still everywhere above storm wave base. High in the Helena are cycles like those of the lower Helena, capped by widespread mudcracks, indicating general drying of the Belt sea at the end of Helena deposition. The base of the overlying Wallace Formation is marked by oolite beds recording transgressing beaches of the Wallace sea followed by siliciclastic cycles with pinch-and-swell couples in their lower parts that thin upward to pinch-and-swell couplets, even couplets and microlaminae in their upper parts. They were probably deposited in a larger, more open body of water, but still on a flat floor that was everywhere above storm wave base. The lens of pinch-and-swell cycles in the Helena, centered along the Tertiary Hope fault, may record Proterozoic down-to-the-east movement along a coincident northwest-trending basement fault . Thickening of both the Helena and Wallace formations south of Jocko line may record down-to-the south subsidence along the Jocko line, as in the Revett Formation. The abrupt westward shift of pinch-and-swell cycles in the Wallace Formation over thinner laminated cycles of the Helena Formation in northern Idaho probably records westward tectonic expansion of the Belt basin during Wallace deposition.