Sequence architecture of the cavi unit, Chasma Boreale, Mars

The principles of terrestrial sequence stratigraphy provide a proven approach for evaluating the cavi unit where it transitions into the north polar layered deposits (NPLD). Three cavi unit outcrops at the head of Chasma Boreale were examined using high resolution imagery and topography and interpreted in a sequence stratigraphic framework. Lithofacies present in the cavi unit were identified and consist of (1) dark lithic material, (2) dark material with icy foresets, (3) icy cavi, (4) isolated dunes, (5) dirty ice, and (6) layered ice. These lithofacies represent a progression in the dominance of aeolian to cryosphere processes and have analogs in the modern Chasma Boreale depositional environment. Parasequences were defined as aeolian units bounded by outcrop-scale unconformities, permafrost layers, and layered ice deposits. The northern outcrop is dominated by sandy lithofacies, with large bodies of sand also dominating the northern parts of the central and southern outcrops. In the central and southern outcrops, sandy lithofacies yield southward to icier accumulations. Concurrently, vertical trends in lithofacies distribution indicate an increasing prevalence of ice deposits through time. Considered across all three outcrops, the parasequences form a set that expresses the northward retreat of the cavi aeolian system coincident with transgression of an ice sheet originating in Gemini Lingula. This retrogradational stacking pattern of cavi aeolian parasequences with gradual transgression of the NPLD environment implies that the transitional cavi unit and NPLD form a single sequence that may correspond to allogenic forcing by orbital parameters. The dynamics between aeolian and cryosphere processes revealed at the parasequence scale indicate that preservation of the stratigraphic record is driven by the ice transgression, suggesting that during episodes of ice sheet collapse, the north polar sedimentary environment reverts to one dominated by aeolian erosion and reworking, as represented by the basal cavi unit.