RAIN SHADOW FORESTS OF THE ANCESTRAL ROCKIES; THE IN SITU BURIAL OF CALAMITES GROVES, PENNSYLVANIAN HERMOSA GROUP, SOUTHWESTERN COLORADO

A unique locality with the in situ burial of Calamites cf. gigantus floodplain forests from western Pangea occurs in outcrops of the Pennsylvanian (lower Desmoinesian) Hermosa Group north of Durango, Colorado in the southern San Juan Mountains. These external and internal casts occur within the updip portion of the tectonically driven Sequence 1 of Gianniny and Miskell-Gerhardt (2009). The Calamites stems and rhizomes occur within 5-70cm thick, poorly sorted, fining upward beds of lithic arkose which range from conglomeritic sandstone to fine upper micaceous sandstones. Trough cross bedding and rare lateral accretion surfaces suggests a braided river flood plain environment with coarser flood event packages. There are multiple fining upward events within the casts which range from fine lower to very coarse upper sandstones. Event sedimentation is also evident in up to six Calamites bearing beds which occur in this 8-10m thick interval. Within this entire interval grain size and bed thicknesses decrease upwards.

Within the study area there are at least twenty five stem casts, rhizomes and impressions of Calamites. In situ stems range from 6–80cm tall with diameters ranging from 5-20cm. Rhizomes are preserved below 8 of these stems, and there are several examples of multiple stems arising from a common rhizome. Prostrate impressions of Calamites also occur on bedding planes. Internodal distances on the stems range from 3-20 cm. Inconsistent internodal length may suggest variable duration of growth seasons and moisture availability (Taylor et al., 2008), or timing between inundation events. No fruiting structures, lateral branches, or signs of paleosol development were observed.

This locality was very close to the Grenadier Highlands (Baars, 1966) of the Ancestral Rockies uplift and on the margin of the Paradox basin in western Pangea. Our data suggest that even in the rain shadow of the Ancestral Rockies, on the downwind side of Pangea, there was sufficient moisture to support dense growth of Calamites forests in the floodplains of braided rivers. In contrast with the coals of the underlying Atokan Pinkerton Trail Formation, this lower Desmoinesian paleoflora and the associated sediments suggest a well drained environment, with seasonal rainfall on these braid plains in the shadow of the Ancestral Rockies.