The Mesozoic stratigraphy of the Domeyko Cordillera includes two principal subdivisions: Jurassic to Lower Cretaceous back arc sedimentary and lesser subaqueous volcanic rocks, and Late Cretaceous to Paleocene back arc terrestrial sedimentary and volcanic rocks. These strata comprise the Precordilleran fold and thrust belt that occurs along the western margin of the Collahuasi district and is best exposed along the Quebrada Guatacondo (Fig. 3.18; Amilibia and Skarmeta, 2003; Amilibia et al., 2008). These rocks have been extensively studied by previous workers (e.g., Perelló and Muller, 1984; Smoje, 1989; Orrego G., 1992; Charrier and Muñoz, 1994; Pichowiak et al., 1994; Prinz et al., 1994; Tomlinson et al., 2001a) and only a brief review of that work is provided here.
3.10.1 Quehuita Formation
The name Quehuita Formation is used here to refer to all marine sediments of the Tarapacá basin (e.g., Prinz et al., 1994). The basin was initiated during the Late Triassic as the back-arc to the La Negra volcanic arc (e.g., Pichowiak et al., 1994) and continued to accumulate marine sediments throughout the Jurassic. Smoje (1989) provides the most detailed investigation of the Jurassic stratigraphy in the Collahuasi district and defines five “formations” that are here treated as members in accordance with Tomlinson et al. (2001a). Between 20º30’S and 21º30’S the Quehuita Group is approximately six kilometres thick and spans Liassic to Kimmeridgian time (Smoje, 1989). It is structurally juxtaposed against the Peine Group along the Domeyko Fault (Smoje, 1989; Tomlinson et al., 2001a; Amilibia and Skarmeta, 2003; Amilibia et al., 2008). The basal section that records transgression in the nascent Tarapacá Basin (Sama Formation: Orrego, 1992) is not known in this area. Age constraints among the Jurassic stratigraphy are typically assigned on the basis of fossil assemblages that are dominated by fish and ammonoid species (Bogdanic and Chong, 1985; Prinz et al., 1994; Tomlinson et al., 2001).
Copaquire Member (Jqc. ?Liassic). The Copaquire Member crops out along the Quebrada Copaquire, east of the Domeyko fault zone (Figs. 3.4, 3.18). It comprises siliceous grey marine shales and minor marls that attain maximum thickness of 570 m. These are overlain by up to 140 m of intercalated coarse-grained quartz sandstones and lesser black shales (Smoje, 1989).
Aquiuno Member (Jqa. ?Dogger). The Aquiuno Member conformably overlies
the Copaquire Member and occurs throughout the upper Quebrada Guatacondo, west of the confluence with Quebrada Copaquire (Fig. 3.18). It comprises a basal ~75 m-thick sequence of marine bioturbated black shales overlain by almost 700 m of massive black shales, siltstones and minor intercalated quartz sandstones (Smoje, 1989).
Quehuita Member (Jqq. ?Callovian). The Quehuita Member (in the sense of
Smoje, 1989) conformably overlies the Aquiuno Member and is best exposed in anticlinal cores in the Quebrada Sipuca southwest of the Collahuasi district. It comprises a laterally extensive basal conglomerate and up to 2,000 m of marine micritic limestones and subordinate siltstones that increase in abundance up-section (Smoje, 1989).
Figure 3.18. Distrubtion of the Jurassic Quehuita Formation in the greater Collahuasi district. Light grey area depicts the extent of Peine Group rocks in the district. Black linework indicates major faults, and dashed blue lines are selected drainages. Landmarks mentioned in the text are indicated. Abbreviations: FS = fault system; km = kilometres. Grid divisions are 2 kilometres.
Majala Member (Jqm). The Majala Member conformably overlies the Quehuita Member and occurs in anticlinal cores exposed near the Guatacondo village (west of the area shown in Fig. 3.18). It comprises a sequence of intercalated green-grey marine shales, marls and fine sandstones that is commonly a few hundreds of metres thick, but reaches 1,600 m thick locally (Smoje, 1989).
Guatacondo Member (Jqg. ?Kimmeridgian). The Guatacondo Member records
terrestrial sedimentation in the Tarapacá Basin, and disconformably overlies the Majala Member. It comprises ~1,500 m of intercalated red shales, marls and quartz sandstones, overlain by ~1,300 m of laminated red-brown shales that crop out widely in the vicinity of the Guatacondo village (Smoje, 1989). Laminated red sandstones that unconformably overlie the Peine Group in the Quebrada Copaquire area (Fig. 3.18) are tentatively correlated with the Guatacondo Member.
A sequence of laminated red sandstones known informally as the Chacarilla Beds occurs approximately 20 kilmetres north of Guatacondo (Galli, 1957). These are also correlated with the Guatacondo Member as they preserve various late Jurassic sauropod footprints (e.g., Moreno et al., 2000)
3.10.2 Cerro Empexa Formation (Ktei)
The Cerro Empexa Formation (Galli, 1957) comprises green-grey to black andesitic terrestrial sediments and subaerial lavas that record a migration of the Late Cretaceous volcanic arc toward the back arc. Detailed stratigraphy and paleogeographic reconstructions are provided by Tomlinson et al. (2001a), and radiometric ages compiled therein range from 63.8 – 72.2 Ma. In the Collahuasi district the Cerro Empexa Formation occurs as fault-bound belt that is limited on its eastern margin by the Domeyko fault system (Fig. 3.19). Massive fine grained blue-grey to green-black andesites and lesser boulder conglomerates ascribed to the Cerro Empexa Formation crop out along the western side of the Quebrada Ornajuno. 3.10.3 Tolar Formation (Ktes)
The Tolar Formation (Maksaev, 1978) is a sequence of red-brown terrestrial clastic sedimentary rocks that unconformably overlie the Cerro Empexa Formation
stratigraphy and mapping between Collahuasi and Chuquicamata, Tomlinson et al. (2001a) interpret that the Tolar Formation was deposited as fluvial infill of localised intermontane basins that formed to the east of the proto-Domeyko Cordillera. The Tolar Formation overlies the Peine Group with slight angular discordance. At Collahuasi it occurs predominantly in fault-bound blocks along the Domeyko fault zone, east of Copaquire and adjacent to El Colorado (Fig. 3.19).