HYPER-CRYPTIC THRUST FAULTS IN THE RUMFORD ALLOCHTHON, SADDLEBACK WIND, CARTHAGE, MAINE

The Bald Mountain-Saddleback Wind massif, located in west-central Maine, is a key component in understanding two contentious aspects of Maine Acadian geology: the Rumford allochthon and relationship of the Day Mountain Formation, a thin to medium-bedded quartzite-pelitic schist sequence of presumed Devonian age, to the Littleton Formation and Seboomook Group. Explanations for the origin of the Rumford allochthon vary, with some authors suggesting a regional-scale extensional detachment, with or without thrusting, while others have questioned its existence altogether. The Seboomook Group in western Maine is equally as controversial due to correlation issues with type Seboomook rocks in northern Maine. Recent detailed bedrock mapping of the extensive ledges on Saddleback Wind has led to the recognition of a new marker, the Royal Flush beds, three times repeated on hyper-cryptic thrusts. The southeast-topping Royal Flush beds thin upward in the thrust pile, respectively 2.1 m, 1.3 m, and 1.0 m thick, and northwest-topping limbs are commonly half the thickness of their presumed initially upright counterparts. Asymmetry of tight to isoclinal fold pairs is common with anticlines consistently southeast of synclines, thus supporting previously inferred northwest vergence. Sand/mud ratios in the Royal Flush protoliths are constant, hence the observed upward thinning is more likely due to differential shear during thrusting rather than initial stratigraphic proximal to distal variation in thickness. In progress detrital zircon, x-ray fluorescence spectrometry, and petrographic studies will help to constrain the provenance, age, correlation, and basin geometry of the Day Mountain Formation and its significance within the Rumford allochthon.