Lithofacies and stacking architecture of a middle Pennsylvanian inner platform, bug scuffle member, gobbler formation, Fresnal Canyon, New Mexico
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The Bug Scuffle Member of the Gobbler Formation is part of an extensive Middle Pennsylvanian carbonate platform that accreted on the western edge of the Pedernal Uplift. A section exposed in Fresnal Canyon of the Sacramento Mountains, New Mexico occupies an inner platform position kilometers from the platform margin. The uppermost 173m of the Bug Scuffle was examined in two continuous sections to identify the lithofacies and stratal architectures of this inner platform icehouse system. Two composite sequences were identified based on facies and bedset thickness trends and a sandstone filled incised valley representing the greatest fall in sea level. Only the three uppermost high frequency sequences (HFS) of the lower composite sequence are exposed. These sequences are comparatively thick (20-30m) and are bounded by weak to well developed exposure surfaces. Parasequences and parasequence sets are poorly developed, with lithofacies dominated by deep to high - energy shallow marine lithofacies. The upper composite sequence is composed of at least 11 HFS. Contrary to many described icehouse platforms, exposure surfaces are not developed between the lower HFS such that they might be better described as parasequence sets (PSS). The lowest PSS is dominated by a deep platform crinoidal lithofacies and is considered the Transgressive Sequence Set. The overlying parasequence sets are dominated by phylloid algal and diverse skeletal lithofacies. These thick (15-20m) PSS are overlain by packages of distinctly different lithofacies and architecture that are composed of a phylloid unit, capped by a peloidal - coated grain grainstone and/or a distinctive gastropod lithofacies, and bounded by well developed exposure surfaces. These thin (2-10m) packages represent classic icehouse HFS. This evolution from thick PSS dominated by open marine facies to thin HFS containing significant restricted marine facies with well developed exposure surfaces reflects the long – term loss of accommodation space on the platform and represents the transition from early to late Highstand Sequence Set. This example highlights that a range of stratal motifs may characterize Late Paleozoic icehouse platforms and that classic icehouse HFS may be just one end member within a given platform.