Rapid Inundation of Gale Crater, Mars, by an Expanding Ocean

Abstract

Gale crater, the field site of the Curiosity Rover, is a complex impact crater with a diameter of 154 km located near the equator of the planet Mars. The oldest lithology in the rover’s landing ellipse is the Hummocky Plains Unit (HPU). It consists of conglomerate and breccias that are highly unsorted and poorly bedded. Its fragments have varied lithology, are angular to subangular, and range in size from cobbles to silt. Its basal contact is not exposed but its top surface rises in elevation from the Bradbury landing site at the margin of the crater floor to the Pahrump Hills locality at the foothills of Mt. Sharp near the crater center. As such, deposition HPU could not be attributed to fluvial processes because rivers could not have flown uphill. One possibility is that it formed as an alluvial fan that was sourced from the crater’s central peak. However, the varied lithology of its fragments points to a possible impact ejecta origin associated with formation of Gale crater.

The Murray formation and its time equivalent the Striated Unit cover the HPU with sharp contacts. The Murray formation is a finely laminated mudstone that lacks shallow-water features such as ripple marks and mud cracks indicating that it was deposited in deep-waters below the influence wave actions. The Striated Unit consists of rhythmic layers with features suggestive of deposition as turbidites in deep-waters. Deposition of deep water strata directly over the impact ejecta conglomerate without intervening fluvial and/or shallow-water lacustrine sedimentation suggests rapid and geologically instantaneous inundation of Gale crater by an expanding body of water: possibly the Northern Ocean.

The closest terrestrial analog to Gale crater is the Jurassic strata of Northern U.S. Gulf Coast. Here, laminated deep-water strata of the Smackover Formation overly alluvial fan conglomerate and breccia of the Norphlet Formation with a sharp contact. This is attributed to rapid inundation of marginal basins such at the Mississippi and North Louisiana salt basins by an expanding Gulf of Mexico during the Late Jurassic rapid sea-level rise.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018