FINISHING WHAT DON WINSTON STARTED: PRELIMINARY CORRELATIONS OF THE MESOPROTEROZOIC APPEKUNNY FORMATION, LOWER BELT SUPERGROUP

For over 50 years, Don Winston used sedimentologic models to address stratigraphic issues in the Belt Supergroup. His last great stratigraphic problem was the Appekunny Fm. Willis (1902) defined the formation at Apikuni Mountain in Glacier National Park. Whipple et al. (1984) described five informal members, but could only correlate member 5 across the park. Since 2013, Don, myself, and others measured a section of potentially correlative rocks at Lake Koocanusa, over 120 km away. Preliminary correlations between several partial sections measured by Don and others were improved upon and compared with the type section. The dominant sediment type is hummocky cross-stratified very fine sand/silt (HCS). Members 1 and 2 form the stratigraphic base of the Appekunny Fm. and are almost entirely composed of HCS with nearly no clay, representing reworking of the silt bedload by oscillatory waves. Member 3 contains antidunes, desiccation cracks in silt-clay couplets, and lateral changes in sediment type that suggest deposition by sheetfloods. Member 4 contains fine sand/silt lenses with load structures in muddy couplets. Deposition occurred by oscillatory waves, like in members 1 and 2, but with more clay. Member 5 contains silt-clay microcouplets with desiccation cracks and mud chips, suggesting episodes of suspension settle out in calm water followed by exposure and flooding.

The deep, structurally-controlled basin of the underlying Prichard Fm., and the adjacent carbonate platform of the Altyn Fm., were progressively filled and buried by silt from west-derived storm deposits. The lower members of the Appekunny record deposition in a shallow, storm-influenced lake. Persistent wind may have driven suspended clay to the margin or through a spillway, resulting in widespread deposition of well-sorted silt. The first subaerial exposure of the Belt basin was during deposition of member 3, as sheetflood and playa complexes developed. In members 4 and 5, mud was exposed and deposited as lake levels fell and rose. The overlying Ravalli Group marks widespread and persistent sheetflood and playa deposition. The Appekunny records the progressive filling of a failed rift basin and a gradual transition from more continuous, storm-influenced sedimentation to punctuated sediment discharge into the low-relief Belt basin from all sides.