General Geology of Northeastern Ecuador
A. S. Nieto, Department of Geology, University of Illinois, Urbana
The geology of northeastern Ecuador and present-day physical processes related to geology are greatly influenced by the tectonic mechanisms responsible for the development of the Andes Mountains. Both geology and active physical processes (landsliding, volcanism, erosion, weathering) are complex and varied. The reader is referred to classic works on these subjects (Tschopp, 1953; Lewis et al., 1956; Ham and Herrera, 1963; Feininger, 1975; Hall, 1977; Baldock, 1982a,b; Feininger, 1987; etc.). Oil and mineral exploration has provided the impetus for detailed studies on the geology of NE Ecuador. The following paragraphs draw liberally on the above-mentioned sources.
The Andes have created three geologic and geomorphic zones: (1) the coastal plains (Costa) to the west, (2) the central mountainous area —the Andes (Sierra) themselves, and (3) the eastern lowlands (Oriente) (Figure 2.1, Figure 2.2). Figure 2.1 presents a geomorphic/geologic framework of Ecuador. The Costa is a region of low relief and low elevation W of the Cordillera Occidental (Western Cordillera), one of the two major branches of the Ecuadorian Andean Mountains. Much of the ground surface of the Costa consists of Quaternary volcanic and alluvial soils that may be unstable under earthquake loads. However, the energy of the March 5, 1987, earthquakes had dissipated to insignificant levels by the time it reached the Costa. Therefore, this region is not discussed here. Only the geology of the eastern two-thirds of Ecuador (the Sierra and the Oriente) is described in the remainder of this chapter, because this was the area most seriously affected by the earthquakes.
The Sierra is bounded on the W by a suture zone (Jubones Fault) that defines the eastern edge of the Costa and on the E by the back-arc fold-and-thrust belt of the Oriente Province (Figure 2.1). The Sierra traverses the
length of the country and is only about 150 km wide, much narrower than the rest of the Andes. Three geologic and geomorphic zones exist within the Sierra: the Cordillera Occidental (Western Cordillera), the Inter-Andean Valley, and the Cordillera Real (Eastern Cordillera).
The origin of the Cordillera Occidental has been given at least two interpretations. Baldock (1982a) interpreted the zone as a sequence of volcanic-arc sediments (the Macuchi Formation), which were deposited in the Upper Cretaceous to Eocene and were tectonically emplaced at a later time. The basement is continental crust except in the extreme N. Feininger (1987) also interpreted the sediments as volcanic in origin. However, high Bouguer gravity anomalies throughout the Costa and the Cordillera Occidental led Feininger to interpret the entire area W of the Inter-Andean Valley to the N and the upper Amazon Basin to the S as allochthonous terrane underlain by oceanic crust.
The Macuchi Formation consists of a thick sequence of pillow lavas and andesitic volcaniclastic deposits. It is overlain by Paleocene and Eocene volcaniclastic and marine sediments. In some areas, Miocene sediments overlie the Macuchi Formation. Small late Tertiary intrusive bodies intersect the Macuchi Formation. Neogene to Quaternary volcanic deposits obscure the Macuchi at higher elevations of this zone.
The Inter-Andean Valley is a graben situated between the two Cordilleras (Figure 2.1). Intermontane basins that occupy this region are more prevalent in the N and become smaller and less continuous to the S. These high valleys (2,500 to 3,000 m in elevation) are filled with Quaternary sedimentary and pyroclastic deposits. The most important of these volcanic deposits is volcanic ash known as “cangahua.” This ash, of aeolian origin, is fine-grained, largely unstratified, and weakly cemented. At times it resembles loess, or slightly cemented, highly porous sandstone. The cangahua is prone to slope failure.
The Cordillera Real is bounded on the W by the Inter-Andean Valley and on the E by the Sub-Andean Zone. Paleozoic, and perhaps older, metamorphic rocks are dominant in this region. These metamorphic rocks were probably formed during a Caledonian orogenic event (Baldock, 1982a). Subsequent orogenic events, including the Laramide and Andean orogenies, likely affected the rocks of the Cordillera Real. Lithologies present in the region include a thick sequence of Paleozoic muscovite-biotite schists and a sequence of mica schists and chlorite schists (Llanganates Group). Isoclinal folding in these metamorphic rocks has been observed in only a few areas. Crenulation cleavage that overprints the folding may imply a subsequent orogenic event. The majority of fracturing is the result of effects of the Andean orogeny and of Neogene uplift. The region is sporadically covered by Quaternary volcanic rocks (lavas) and sediments (cangahua), which usually are unconformable with underlying metamorphic rocks.
The Oriente (Figure 2.1, Figure 2.2) consists of two distinct structural zones and physiographic provinces: the Oriente Basin and the Sub-Andean Zone. Physiographically, the Sub-Andean Zone consists of foothills rising to elevations of up to 2,000 m. East-flowing rivers have deeply dissected these foothills. The climate varies from tropical in the eastern portions to subtropical in the higher western reaches. Rainfall is high everywhere; as a consequence, rates of weathering are generally high. The Sub-Andean Zone, which borders the Cordillera Real (Figure 2.1, Figure 2.2), is a back-arc fold-thrust belt tectonically associated with the Andes (Baldock, 1982a). Two folded features, the Napo uplift to the N and the Cutucu uplift to the S, are separated by the Lorocachi arch. Reventador Volcano is located on the Napo uplift.
The Oriente (or Amazon) Basin lies east of the Sub-Andean Zone. This is a gently warped basin that represents a more stable tectonic history than that of the Sub-Andean Zone. The stratigraphy of the two zones is similar.
The Guyana Shield (Precambrian crystalline rocks) makes up the basement of the Oriente Basin. In the early Paleozoic, part of the Oriente underwent transgression and sedimentation. The Caledonian orogeny affected the Oriente region only by shifting the axis of sedimentation eastward. Paleozoic lithologies include shales and quartzitic sandstones of the Devonian Pumbuiza Formation and limestones of the Carboniferous Macuma Formation. Three tectonic events during the Mesozoic and late Tertiary had little tectonic effect on the Oriente. Similarly, the Laramide orogeny of late Eocene and Oligocene had a minimal influence on the Oriente. Basin sedimentation resulted in deposition of fresh-water and terrestrial lithologies. Middle Jurassic to late Cretaceous redbeds (Chapiza Formation) and clastics and pyroclastics (Misahualli Member of the Chapiza Formation) underlie the Cretaceous Hollin-Napo-Tena Formations. In the Reventador area, the Mishualli Member increases in thickness and is predominantly volcanic. Rock types associated with the Hollin-Napo-Tena group include quartzitic sandstones (Hollin Formation), which are reservoir rocks for petroleum in NE Ecuador. The overlying Napo Formation consists of shales, limestones, and sandstones, all of marine origin. The sandstones may also be reservoir rocks. The Tena is composed of redbeds and shale. Redbeds and some sandstones and clays represent early Cenozoic deposition.
The major deformation of the Sub-Andean Zone took place in the late Miocene and Pliocene. Overthrusting and uplift during this time were responsible for the present segregation of the two zones of the Oriente. Major thrust faults associated with this event generally trend NNE to SSW (Figure 2.1 and Figure 2.2). Two additional sets of faults or fractures have been observed in the Reventador Volcano area of the Napo uplift (INECEL, 1987). There is an indication of low-grade metamorphism associated with some of the major (NNE to SSW) thrust faults in the Reventador area of the Napo uplift.
Quaternary clastic sedimentation in the Oriente includes a variety of deposits, from lavas and pyroclastics of all grain sizes to colluvial/alluvial materials (piedmont fans) to alluvial fills.
Two major areas of Quaternary volcanism occur in the Sub-Andean Zone (Hall, 1977) (Figure 2.1). Sumaco Volcano (20 km SE of Baeza, Figure 1.1) deposited alkaline undersaturated basalts. Reventador Volcano ( Figure 1.1, Figure 1.3) exhibits a more typical suite of lithologies, including andesitic basalts and a significant amount of pyroclastic and lahar deposits. Both volcanos are situated on the Napo uplift and overlie Cretaceous rocks. While Sumaco and Reventador volcanoes are both considered active, Reventador alone has undergone historic and frequent volcanic activity. The morphological evolution of Reventador Volcano began in the Pliocene. The original volcanic cone (Paleo
Reventador I) collapsed initially (perhaps in the Pliocene); the remains of this collapse created the “Complejo Volcánico Basal” of INECEL (INECEL, 1987). Activity resumed in the Holocene, and culminated with the collapse of another cone—Paleo Reventador II—about 20,000 years ago; this second cone has been referred to as the “Paleo Reventador” by INECEL. This second collapse dammed the Coca River and caused the deposition of 20 m of lacustrine sediments that have been dated radioactively by INECEL. The material from the two collapses reached the right bank of the Coca, creating terranes of volcanic debris that are present there today.
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