A Reference Model for Martian Deltas: Links Between Delta Topsets and Climate-Driven Flows in the Upper Baker River Basin, Chile

Deltas are an important archive of past environmental changes. Nearly 200 putative deltas have been identified on Mars. Rover observations of two paleolacustrine sites at Gale and Jezero craters is providing new constraints on the nature and depositional conditions of the fan deposits. Terrestrial analogs are critical for accurate lacustrine reconstruction. Relatively under-considered for martian deltas are cold climate deltas, especially coarse-grained examples. The Patagonian upper Baker River basin is an excellent natural laboratory with a well-established climate and flood record that can be linked to the exposed deltaic architecture.

Chilean Lake General Carrera is an elongated basin (130 km x 20 km) east of the Andes Mountains that extends into Argentina where it is called Lake Buenos Aires. Over a dozen deltas at LGC developed during a series of lake lowering events since the breakup of the Patagonian Ice Field ~15 ka. The deltas have a classic Gilbert form with a braided river on the top surface and a steeply inclined front. The last major lake lowering event, from 320 masl to present level 201 masl, occurred after 6.2 ± 0.8 ka. This study describes the sedimentary architecture for the lowest exposed delta terrace outcrops at five sites, with ties to climatically-induced flow conditions.

The youngest perched deltas exhibit inclined foreset beds overlain by 10-30 m thick horizontally stratified topset strata with a sharp truncation surface. Topset beds are dominantly comprised of rounded pebble and cobbles (minor sand), with interbedded meter-size boulders (5-30% by volume). Seasonal floods recorded in three stream gauges have maximum daily discharge sufficient to transport cobble-size clasts, but threshold conditions for boulders are only achieved in glacial outburst events. Regionally, such catastrophic floods had a multi-century recurrence interval until the last 2 ka.

The glacio-fluvial LGC delta model should be considered for select Mars delta reconstruction, and has ramifications on flood triggering mechanisms, formation timescales and associated climate conditions. Importantly, if ice-release events played a role, the total duration for delta formation is significantly lengthened.