Abstract

Within the Mesozoic fold-and-fault belt of west-central Peru between Lake Junín and Huancayo, the Pucará Group is subdivided into three formations. The rocks were originally described by Harrison (1944) and the formations were named from near Huancayo by Mégard (1968). They are: (1) Condorsinga Formation (top): limestones, shaly, sandy, bioclastic, and cherty; thickness is as much as 2900 m; (2) Aramachay Formation: bituminous shales and sandstones, cherts, and phosphatic rocks; thickness is as much as 600 m; (3) Chambara Formation: limestones, shaly, bituminous, and cherty; thickness is up to 1500 m.

Although relatively lacking in fossils diagnostic of age, the Chambara Formation is assigned a late Triassic age (Norian-Rhaetian); it is probably correlative with most of the Pucará Group of Cerro de Pasco area (Jenks, 1951; Haas, 1953). The other two formations are Early Jurassic in age (Aramachay: Hettangian in part to middle Sinemurian; Condorsinga: upper Sinemurian to upper Toarcian).

The Pucará Group represents a miogeosynclinal sequence, unconformably underlain by red clastic and volcanic rocks of the Permian Mitu Group, and disconformably overlain by clastic rocks of the Lower Cretaceous Goyllarisquisga Group or by red beds of the Middle Jurassic Cercapuquio Formation.

The Chambara Formation indicates a regionally uniform and relatively deep environment of deposition. The Aramachay represents predominantly an accumulation of fine clastics and much organic material in a deep marine basin, which was followed by, and was probably preceded by, deposition of diverse sedimentary facies revealing shifting and laterally variable marine environments. The limestone beds of the Condorsinga Formation show relatively uniform and shallower marine sea-floor conditions. Rates of deposition for the three formations individually, and for the group as a whole, in cm/1,000 yrs are: Chambara: 15; Aramachay: 5.7; Condorsinga: 12.5; Pucará Group, cumulative rate: 9.7.

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