Geology of Hebes Chasma, Mars: 1. Structure, Stratigraphy, and Mineralogy of the Interior Layered Deposits

Abstract

Hebes Chasma is an 8-km deep, 126 by 314 km, isolated basin that is partially filled with massive deposits of water-altered strata called interior layered deposits (ILDs). By analyzing the ILD's structure, stratigraphy, and mineralogy, a depositional history of Hebes Chasma is interpreted. Three distinct ILD units were found and are informally referred to as the Lower, Upper, and Late ILD. These units are distinguished by their layer thicknesses, layer attitudes, mineralogies, and erosional landforms. The Lower and Upper ILDs comprise the chasma's 7.5-km tall, 120 by 43 km, central mound, and the Late ILD is located in the valley between the central mound and the chasma's northern wall. A horizontal unconformity separates the Lower and Upper ILDs, and layer attitudes revealed large-scale shallow folding within the Lower ILD. All ILDs are characterized by both monohydrated and polyhydrated sulfates signatures. Erosional landforms such as hummocks, polygons, and debris flows suggest past glacial activity within the chasma. A scenario involving several ash fall events during various stages of chasma formation is proposed as the dominant setting throughout Hebes' geologic history.