Identification of the expression of earthquake-induced surface flooding by groundwater using detailed regolith mapping at the buried Atlántida Deposit, northern Chile by A. E. Brown, P. A. Winterburn, and T. Bissig Geochemistry: Exploration, Environment, Analysis Volume ():geochem2018-064 May 24, 2019 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Map of northern Chile showing that large areas of the country are covered by gravels. Map of northern Chile showing that large areas of the country are covered by gravels. Outcrop in grey and alluvial/colluvial cover in white. Black circles indicate study areas: 1, Atlántida; 2, Papas; 3, Mastodonte; 4, Viento. Black stars indicate major copper porphyry clusters: 1, Collahuasi; 2,Chuquicamata; 3, Spence; 4, Gaby Sur; 5, Escondida. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Schematic diagram of the response of alluvial fans to fault movement and the formation of saline pockets. Schematic diagram of the response of alluvial fans to fault movement and the formation of saline pockets. (a) Deep-seated bedrock faults extending from depth to surface create a fracture zone of high permeability in overlying alluvial fans. (b) Fault action affects geomorphology of the alluvial fan, exposing the calcrete horizon. Saline groundwater transported through high permeability zones by pumping during seismic activity. Water pools at the surface. (c) Calcrete horizon exposed on the side of the uplifted fan. Highly saline pockets of material form as a result of evaporation of pools of groundwater. Saline pocket sites are not preserved in erosional channels. (d) Field photograph showing surfaces containing saline pockets outlined by dashed yellow lines and young erosional channel where saline pockets are not preserved (outlined by white dashed line). A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

(a) Regolith landform map of Atlántida area with underlain DEM (z = 5). (a) Regolith landform map of Atlántida area with underlain DEM (z = 5). Structures extending to surface and affecting alluvial fan morphology (red dashed lines) are identifiable using geomorphic markers of faulting, regolith mapping and drone imagery. Structures mainly trend NE and north–south. A major NE-trending fault forms a valley striking NE–SW where gravel cover is the deepest. (b) Imagery from ArcGIS of Papas prospect area with structures (red dashed lines) identifiable by geomorphic markers. Top image shows a large east–west-trending structure which has significantly uplifted alluvial fans across the area (up to 5 m). Bottom image shows the trace of structures following geomorphic markers using the imagery. Dark red-coloured alluvial fans in the imagery indicate surfaces with clasts covered in Mn-oxide crusts, indicative of a relatively older alluvial surface. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Drone imagery of geomorphic markers of faulting. Drone imagery of geomorphic markers of faulting. Surfaces with saline pockets are outlined in black. (a) Sharp, straight-sided alluvial fan fault scarp created by NE-trending fault. Calcrete horizon exposed by fault action (white coloured) is clearly visible along the fault scarp. Field photograph (right) of exposed calcrete horizon, weathered into clasts of calcrete (inset photo). (b) Calcrete horizon uplifted to the surface by fault activity clearly visible in imagery (left) compared to field observation (right) which is subtle and easily missed without the aid of imagery. (c) Erosional channels of young, reworked material are diverted by faulting in imagery (left) and in the field (right). North–south- and NW-trending structures divert younger alluvial material infilling the valley, creating abnormally sharp-sided older alluvial fans. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

(a) Drone imagery with stretched colour bands. (a) Drone imagery with stretched colour bands. Saline pocket surfaces appear darker (outlined by yellow dashed lines) and are distributed along structural trend. (b) Saline pockets (outlined by yellow dashed lines) are distributed along structural trend and appear on the surface as dark, slightly depressed surfaces. Shovel for scale. (c) Saline pocket surface (outlined by dashed yellow line) on the side of an alluvial fan, demonstrating red–orange-coloured material with less clast content on the surface. Field notebook for scale. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Conductivity measurements over 10 m across a saline pocket and extending into background alluvial material. Conductivity measurements over 10 m across a saline pocket and extending into background alluvial material. High conductivity values are highly localized and drop to background levels within 50 cm of the saline pocket. Material sampled at 5 cm depth and sieved to <1 mm fraction. Conductivity measured with a field portable conductivity meter. Measurements above 20 000 µS were calculated using dilution factor of 4 for measurement of a 1∶1 slurry by volume. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Histograms of conductivity (µS) in saline pocket samples (grey) and background alluvium (black) from all properties, shown separately. Histograms of conductivity (µS) in saline pocket samples (grey) and background alluvium (black) from all properties, shown separately. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

(a) Na plotted against Cl by deionized water leach; and (b) histogram of pH of saline pocket samples (n = 138, grey) and background alluvium (n = 196, black) from Atlántida. (a) Na plotted against Cl by deionized water leach; and (b) histogram of pH of saline pocket samples (n = 138, grey) and background alluvium (n = 196, black) from Atlántida. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Saline pockets with varying surface expression at the Mastodonte and Viento prospects. Saline pockets with varying surface expression at the Mastodonte and Viento prospects. As a result of the increased aridity of the northern Atacama Desert, salt is preserved on the alluvial surface at these localities. Signs of pooled water include fewer clasts on the surface (a) and slight puddle-like depressions covered with salt (b). Saline pockets can also be present on the surface as simple white patches, potentially preserving a ‘feeder system’ of fractures to the surface (c). A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Field photographs of saline pockets, all outlined by yellow dashes lines. Field photographs of saline pockets, all outlined by yellow dashes lines. (a) Vesiculated, easily penetrable fine-grained material within the dashed lines. Inset photograph demonstrates vesiculated nature of crust. (b) Saline pocket at the Papas prospect, plastic shovel for scale. (c) Slightly depressed saline pocket surface showing lesser clast content on the surface with small centres of soft, fine-grained material (proximal to the geological hammer). (d) Larger saline pocket surface from the Atlántida demonstrating the distinct changes on the surface between the saline pockets and surrounding alluvium. Field notebook for scale. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

Particle size fractions of saline pocket samples (n = 3) and background alluvium (n = 3) from Atlántida. Particle size fractions of saline pocket samples (n = 3) and background alluvium (n = 3) from Atlántida. Photographs of size fractions from one saline pocket and one background alluvium sample are shown below the graph. All samples were sieved to 180 µm in the field prior to transport to Vancouver, therefore particle size analysis ranges from >150 to <25 µm. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

(a) Left, typical alluvial surface at Atlántida dominated by cobble surface. (a) Left, typical alluvial surface at Atlántida dominated by cobble surface. Right, cleared surface (1–4 cm) revealing sandy, low salinity material which cannot be easily penetrated with a plastic shovel. (b) Left, surface of a saline pocket. Right, cleared surface revealing highly saline, very fine, deep red-coloured material which has few to no clasts and is easily penetrable by the plastic shovel. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

XRD analysis of background alluvium sample and saline pocket sample from 5–10 cm depth. XRD analysis of background alluvium sample and saline pocket sample from 5–10 cm depth. Samples were collected less than 2 m apart. Following the standard powder diffraction smear method, samples were analysed by Bruker D8 Focus (0–20, LynxEye detector) diffractometer using cobalt as the X-ray target at the University of British Columbia. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG

(a) Left, plan view of a saline pocket with a fracture on the surface at Atlántida. (a) Left, plan view of a saline pocket with a fracture on the surface at Atlántida. White-dashed line indicates trace of where small trench was dug. Right, section view the small trench (40 cm depth) dug across the fracture. The trench reveals a fracture that extends from shallow depth to surface at an oblique angle to the trench-wall (outline by black dashed lines). (b) Left, vertical fracture cutting through overlying gravels at the Papas property. Right, a saline pocket is present on top of the fracture on the alluvial surface. A. E. Brown et al. Geochemistry: Exploration, Environment, Analysis 2019;geochem2018-064 © 2019 The Author(s). Published by The Geological Society of London for GSL and AAG