Tectonic Implications of Late Cenozoic Sedimentation from the Coastal Cordillera of Northern Chile (22-24"S)

Journal ofrhe Geological Sociery, London, Vol. 152, 1995, pp. 51-63, 11 figs, 1 table. Printed in Northern Ireland Tectonic implications of Late Cenozoic sedimentation from the Coastal Cordillera of northern Chile (22-24"s) A. J. HARTLEY' & E. J. JOLLEY2 'Production Geoscience Research Unit, Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB9 2UE, UK 'BP Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB2 OPB, UK Abstract: Late Cenozoic sediments from northern Chile (22-24"s) are exposed on the coastal plain between the Coastal Cordillera to the east and tilted fault blocks of the Mejillones Peninsula to the west. During the mid-Miocene to Pliocene (?up to mid-Pleistocene) a shallow marine basin developed unconformably over basement. Sedimentation was initially restricted to a small half-graben on the western margin of the Mejillones Peninsula during the Miocene. Expansion of the basin during the early Pliocene resulted in widespread shallow marine sedimentation across the study area. Alluvial, aeolian and beach sediments were restricted to the basin margins, where sediment was supplied from the Coastal Scarp (a major cliff-line bounding the western margin of the Coastal Cordillera) and isolated(islands) fault blocks. Areas of restrictedclastic input were characterized by carbonate deposition. Marine planation surfaces or terraces (0.5-600m elevation) and associated palaeo-sea cliffs cut intoMiocene-Pliocene sediments and basement rocks, developed along the coastline of northern Chile in the ?late Pliocene to late Pleistocene. A similar age for a number of late Pleistocene terraces (125 OOO years) now at different elevations, suggests that they were cut during interglacial highstands (oxygen isotope stage 5). Variations in terrace elevation are attributed to faulting. Regionalscale uplift (over hundreds of kilometres) of shallowmarine sediments beneath an extensive pre-mid Pleistocene shoreline is considered to be related to variations in subduction zone geodynamics. The favoured hypothesis is that of aseismic ridge subduction. Keywords: Chile, Upper Cenozoic, tetonics, sedimentation, marine terraces. The late Miocene to Recent uplift of the centralAndean of fault-relateduplift resulting from active N-S-trending Pacific margin of South Americais recorded by the faults which cross-cut thepeninsula and glacio-eustatic development of a number of marine terraces and exhumed sea-levelfluctuations. Descriptions of these uplift related shallowmarine and continental sediments of Miocene to phenomena are restricted to the area around the Mejillones Recent age. However, whilst terrace development has been Peninsula and little attention has been paid to uplift related recognizedfor some time (e.g. Fuenzalida et al. 196S), features north and south of the peninsula. Here we present correlationalong the Pacific marginhas proved extremely data on Miocene to Recent shallow marine sedimentation, difficult. Correlation difficulties havearisen because of terrace, beach ridge and alluvial fan development along the variations in theages, numbers and heights of terraces. north Chilean coast between 22" and 24"s. These data allow These variations are thought to be related to a combination constraints to be placed on uplift mechanisms at both local of fluctuationsin sea-level resulting from the Quaternary and more regional scales. glaciation (e.g. Radtke 1987) superimposed on areas of the Pacific marginsubjected to differential uplift. This Geological setting differentialuplift is likely to berelated to changes in the The Coastal Cordillera forms a prominent topographic high geodynamic configuration of the subduction zone (e.g. Flint throughout northern Chile with an average height of 1km et al. 1991; Machare & Ortleib 1991). Here we illustrate how (locally up to 2 km). The Cordillera is bounded to the east a detailed study of Miocene to Recent sediments from the by the Atacama Fault Zone and to the west by a major cliff north Chilean coastal margin can help to constrain the scales line known as the Coastal Scarp (Fig. 1). The Coastal Scarp at which different uplift mechanismshave affected the extends for over700 km and has previously been interpreted leading edge of the continental margin. The understanding as either an extensional fault (Paskoff 1980) or palaeo-cliff of these uplift mechanisms hasimportant implications for line (Mortimer & Saric 1972). The study area comprises the the tectonicevolution of theCoastal Cordillera and the coastal plain between 22" and 24"s (Fig. 2). In this area the relationship between forearc deformation and subduction. plain shows a gradual decrease in gradient from alluvial fans at the Coastal Scarp to the present day shoreline. At the shoreline, the fans terminate in one or a series of terraces Previous work which variably exposealluvial, beach or shallow marine Previouswork on coastal uplift innorthern Chile has sediments. Between 22" and 23"s the plain increases in width focused onmarine terrace and beach ridge development from SO0 m to 4500 m (Fig. 2). Between 23" and 23'30' the acrossthe Mejillones Peninsula (Herm 1969; Okada 1971; alluvial fans pass westwards from the Coastal Scarp onto the Armijo & Thiele 1990: Fig. 1). These authors suggested that easternmargin of the Mejillones Peninsula (Fig. 2). The terracedevelopment could be attributed to a combination peninsulacomprises a series of fault-bounded blocks 51 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/51/4889907/gsjgs.152.1.0051.pdf by guest on 29 September 2021 52 A. J. HARTLEY & E. J. JOLLEY Fig. 1. Geological map of the Coastal Cordillera of northern Chile between 22 and 24% (modified from Boric er al. 1990), illustrating the main fault patterns and extent of Miocene to Recent shallow marine and associated sediments. Area east of the Atacama Fault Zone left blank for simplicity. Fig. 2. Geomorphology of the Coastal Cordillera between 22 and separatedfrom the Coastal Scarp by alow-lying plain of 243, illustrating the distribution of marine terraces, beach ridges uplifted marinesediments of Miocene to Pleistoceneage and main faults. The coastal plain is defined as being below the (Fig. 1). South of the peninsula (south of 23'30') the coastal 200 m contour. Boxes indicate position of aerial photographs and plain is relatively narrow and similar in morphology to the detailed maps (Figs 5, 8 & 9). CH, Calita Herradura. northern part of the study area. The Coastal Cordillera itself largely comprises andesite- dominated volcanicrocks of the 10km thick Jurassic La Negra Formation(Garcia 1967) andassociated Jurassic Miocene-Recent stratigraphy granodioriticintrusions (Fig. 1). As well asthe igneous FiveMiocene Recentto stratigraphic unitscanbe rocks,fault blocks onthe MejillonesPeninsula andthe recognized onthe coastal margin between 22" and 24"s southernpart of thestudy area include Palaeozoic unconformably overlying older rocks (Fig. 3). (1) Miocene metamorphic rocks and Cretaceous red beds and limestones shallowmarine sediments of the 160 mthick Caleta of the Coloso and El Way Formations (see Ferraris & Di HerraduraFormation (Martinez-Pardo 1980; Krebs et al. Biase 1978; Hartley et al. 1992 forfurther details). Clast 1993). The formation crops out at Caleta Herradura (on the typeswithin thePleistocene and Holocene alluvial fans northwesterncoast of the MejillonesPeninsula) and is indicate an igneous and metamorphic source area. unconformably overlain by the La Portada Formation. (2) Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/51/4889907/gsjgs.152.1.0051.pdf by guest on 29 September 2021 CEN O ZOIC SEDIMENTATION. CHILESEDIMENTATION. CENOZOIC 53 Thickness aspre-mid-Pleistocene. The unit is considered to be Strat.Lithology equivalent in age to the La PortadaFormation. (4) (m) Pleistocene Holoceneto shallowmarine and beach sediments of the Mejillones Formation (Ferraris & Di Biase 1978). The formation is up to 80 m thick and unconformably overlies the La PortadaFormation. The Mejillones Z Formation is exposed over much of the northern part of the 0 MejillonesPeninsula. (5) Pleistocene to Holocene alluvial i=* fan, aeolian and beach sediments (up to 50m thick) which flank the Coastal Cordillera and are equivalentin age to the Mejillones Formation. 0.1 Sedimentology L> *3 Caleta Herradura Formation 0 The formation was defined by Krebs et al. (1993) who also ~n described the sedimentology of the 160 m thick section. The JZ section comprises three main facies associations: (1) a lower unit of well rounded, fossiliferouspebbly conglomerates W withcoarse-graineda matrix interpreted asnearshore reworking of alluvial fandeposits (Krebs et al. 1993), interbedded withshell-rich, cross-stratified coarsegrained sandstones interpreted as wave-dominated upper shoreface 1.64 deposits, (2) fossiliferousmudstones interbedded with cross-stratified sandstonesand pebbly fossiliferous con- glomeratesorganised into coarsening upwards units and interpreted as pulsed offshore bar construction (Krebs et al. 1993)and (3) fine-grainedburrowed and cross-stratified sandstonesinterpreted as middle upperto shoreface ... deposits. Thetop of theformation is capped by a d.. .'.l. diatomaceousmarine mudstone unit up to3m thick 5.2 deposited in an offshore pelagic environment. La Portada Formation The formation was defined by Ferraris & Di Biase (1978) v::... and recentlymodified by Krebs etal. (1993)(Fig. 3). It is ..... .o:. exposed in sea cliffs at La Portadato the north of l Antofagasta where it

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