Skip to Main Content
Skip Nav Destination

Three stratally disrupted chaotic units crop out within a horst in west-central Maine. These mélanges and olistostromes are below, within, and above a 2.4-km-thick sequence of ophiolitic metabasalt. Each is characterized by a predominantly pelitic matrix. Textures of the matrix range from slate through phyllite to very fine grained schist. The chaotic units are differentiated by the structures within the matrix, and the lithology and structures of the inclusions.

The matrix of the Loon Stream mélange is characterized by slaty cleavage. The inclusions are metasandstones and metarhyolites. The metasandstone inclusions exhibit layer-parallel extension and layer-normal shears. This mélange is interpreted to have formed either through (1) slope failure related to volcanic activity associated with the overlying metabasalts, or (2) as a thrust zone associated with the transport and emplacement of the previously erupted metabasalt pile. The first implies that this mélange is an olistostrome, while the second implies a broken formation.

Chaotic units within the Caucomgomoc Lake Formation are olistostromes. These are thin units (3 to 15 m) enclosed within a stratiform sequence of grain flow and turbidite origin. The matrix is slaty. The inclusions are metasandstone, metashale, and rarely, metabasalt. The metasandstone beds exhibit features associated with sedimentary boudinage, including necking, distension, and rotation. The olistostromes are laterally restricted and are interpreted to have been deposited within a channel, such as a canyon, at or near the base of a slope.

Chaotic rocks of the Hurd Mountain Formation warrant interpretation as a tectonically formed mélange that originated through the combined processes of offscraping and underplating. The mélange matrix exhibits an extensive shear cleavage, with widespread cataclasis of the matrix and inclusions. A wide range of sizes, structural features, and mineralogies is displayed in the inclusions. Metabasalt inclusions derived from the underlying Caucomgomoc Lake Formation were combined within the mélange through tectonic plucking during underplating. Later features related to uplift within an accretionary prism include: chevron folds, crenulation cleavage, late-stage syntectonic igneous intrusion, and imbricate thrust emplacement of metabasalt slabs.

The preferred interpreted sequence of formation is as follows. (1) Channelized olistostromes were deposted within the Caucomgomoc Lake Formation. (2) Subduction, once initiated, offscraped and underplated newly deposited trench and trench-slope sediments, as well as detaching, metamorphosing, and tectonically plucking metabasalt rafts and inclusions. Imbricate thrusting of metabasalt rafts culminated in the development of the Hurd Mountain mélange. (3) Finally the tectonically assembled mélange (Hurd Mountain) and ophiolite (Caucomgomoc Lake Formation) were transported, producing the mélange of the Loon Stream Formation. These sequences are interpreted to be related to the development of a Late Cambrian(?) through Early Ordovician forearc. This zone is now recognized as the suture between the Boundary Mountains and Traveler (Gander) terranes.

You do not currently have access to this chapter.

Figures & Tables

Contents

References

Close Modal

or Create an Account

Close Modal
Close Modal