STRATIGRAPHY, STRUCTURE AND METAMORPHISM OF THE CHINA LAKE, ME 7.5' QUADRANGLE

Two formation scale map units dominate the China Lake, ME 7.5'quadrangle. A stratigraphically lower thin bedded metapelite is overlain by thin to thickly bedded metasandstone. Fine – grained biotite granodiorite assigned to the Three Mile Pond pluton crops out in the southwestern corner of the quadrangle. Metamorphic zones increase from northeast to southeast and west. Metamorphic zones range from chlorite to sillimanite.

The lowermost unit, assigned to the Waterville Formation is thinly bedded and ranges in lithology from a slate - rich unit in the north through a variety of fine-grained biotite, garnet, staurolite + andalusite, to sillimanite bearing schists in the south. The overlying unit, assigned to the Vassalboro Formation, is a thin to thick (6 cm - 2 m) bedded quartzo-feldspathic metawacke and granofels. A variety of turbidite – related sedimentary structures are common in lower metamorphic zones in the northern 1/3 of the quadrangle Interbedded with the granofels are subsidiary thin (1-9 cm) metapelites which range from slate to fine-grained biotite (+/- garnet) - bearing quartzo – feldspathic schist. Thin (3 - 8 cm) calcsilicate beds are locally present in southern portions of the quadrangle. A thin rusty weathering unit separates the Waterville from Vassalboro formations. A separate identical rusty weathering unit occurs at a stratigraphically higher position within the Vassalboro. Additionally, thin rusty weathering beds are local within both the upper Waterville and lower Vassalboro. This rusty unit is commonly a very fine- to fine - grained quartz, plagioclase, muscovite, biotite, (+/- sillimanite), pyrite, pyrrhottite, graphite bearing metasiltstone and schist. Fine-grained, rusty weathering quartz – rich metasandstone beds are locally present in this unit.

Quadrangle structure is dominated by a series of northeast – trending isoclinal folds with a regional syncline in the central portion of the quadrangle. In the southeastern portion the Dearborn Brook Fault strikes northeast – southwest. Exposures of the fault include uncommon pseudotachylyte together with indicators of dextral strike slip.