Geologic map of the polar regions of Mars

The map units are arranged according to their occurrences and associations as outlined in the correlation chart. The origin and composition of many units are obscure or controversial, but their identification is based on objective descriptions of morphologic characteristics visible on Viking images. Many of the units and their type areas occur in equatorial regions (Scott and Tanaka, 1986; Greeley and Guest, 1987); unit symbols, names, and groupings already established there are used here. Northern plains assemblage Materials deposited in widespread sheets on northern plains. Boundaries between rock units commonly not well defined, in places indicated by dashed contact. ARCADIA FORMATION—Smooth, sparsely cratered; lobate fronts visible in places. Embays all neighboring units. (Members 2, 4, and 5 (units Aa 2 , Aa 4, and Aa 5) mapped to south, not present in map area.) Interpretation: Lava flows and sediments from local sources Aa 3 Member 3—Forms isolated patches along edge of map area between long 170° and 180° Aa 1 Member 1—Forms low-lying plains surrounding Alba and Tantalus Fossae Aps SMOOTH PLAINS MATERIAL—Forms two areas of smooth, sparsely cratered plains north of crater Lyot near edge of map area (long 315° to 340°) and near knobby, undivided material at long 193°. Interpretation: Probably of diverse origin, but may primarily consist of eolian deposits VASTITAS BOREALIS FORMATION—Subpolar plains deposits. Type areas designated in western equatorial region (Scott and Tanaka, 1986) Hvm Mottled member—Characterized by high-albedo crater deposits superposed on low-albedo, smooth-plains deposits; occurs along more than half of edge of map area; gradational with other members, particularly with knobby member. Interpretation: Lava flows, possibly erupted from fissures and small volcanoes, or alluvial or eolian deposits. Mottled appearance due either to relatively fresh, light-colored material exposed during impact-crater excavation or to high-albedo eolian debris trapped within crater ejecta Hvg Grooved member—Marked by grooves forming polygonal pattern; polygons commonly 5 to 20 km across; small patch at lat 80° N., long 60° in mouth of Chasma Boreale; eroded on south side. Hvr Ridged member—Scattered occurrences mostly in western longitudes; gradational with knobby, mottled, and smooth members; in places embayed by Arcadia Formation. Ridges about 1 to 2 km wide and several to tens of kilometers long commonly form polygons 5 to 20 km across; ridge patterns in some southern outcrops are arcuate or concentric, as at lat 56° N., long 173°. Interpretation: Degraded lava flows or sediments; ridge pattern may be formed by …

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