NEW PALYNOLOGICAL DATA FROM MID-CARBONIFEROUS HEATH AND TYLER FORMATIONS CONSTRAIN TIMING OF CLIMATIC CHANGES AND SEDIMENTATION, BIG SNOWY TROUGH, CENTRAL MONTANA, USA

Palynology from four cores and one outcrop (55 samples) reveals the Heath Formation is largely mid- to late Visean, the Tyler (Stonehouse Canyon and Bear Gulch Members) is Serpukhovian, and the Cameron Creek Member (upper Tyler) is Bashkirian (Morrowan). These ages mean that the underlying Otter and Kibbey can be no younger than mid-Visean. More detailed analyses indicate that the Van Dusen glacio-eustatic cyclic strata (Knoxisporites triradiatus-K. stephanophorus TS and early Petrotrillites tesselatus-S. campyloptera TC palynozones of western Europe) coincide with mid-Visean glacial episodes in South America. The Cox Ranch and lower Red Hill (TC to R. nigra-marginatus NM zones) correspond to a period of climatic warming and drying whereas the upper Red Hill dolostones and anhydrites (Brigantian Tripartites vestustus-Rotaspora fracta VF zone) are synchronous with widespread warming and glacial retreat in Gondwana. The upper Heath is late Visean-earliest Serpukhovian (VF to Bellispores nitidus-Reticulatisporites carnosus NC zones). The Heath Formation correlates to the Railroad Canyon, Scott Peak, and South Creek Formations in Idaho and a hiatus in southwestern Montana.

The Stonehouse Canyon contains species consistent with the M. trigallerus – R. knoxiae to early I. subtriquetra-K. ornatus zones of early to middle Serpukhovian age (Arnsbergian). This interval with abundant wood and plant material and fluvial deposits correlates to the Lombard of southwestern Montana and the Railroad Canyon, Surrett Canyon, Arco Hills in Idaho and coincides with more extensive glaciations in Gondwana. The Bashkirian Cameron Creek represents seasonally wet and dry conditions with sufficient moisture for development of fluvial systems and correlates with the lower Amsden and Conover Ranch in southwestern Montana and parts of the Bluebird Mountain and Snaky Canyon in Idaho.

The Heath-Stonehouse Canyon sequence represents a pulsed transition from hot and arid to wet and humid equatorial climate conditions coeval with Gondwanan warming and glacial retreat to cooling and glacial advances respectively. These climatic changes are related to a reaction of North American climates to increasing glaciations in Gondwana, closing of the Rheic Ocean, and the northward drift of North America through differing climatic zones.