Borealis Quadrangle - Geologic History

Geologic History

Five periods were postulated by Murray and others (1975) to constitute the history of Mercury’s surface: (1) accretion and differentiation; (2) terminal bombardment; (3) formation of the Caloris Basin; (4) flooding of that basin and other areas; and (5) light cratering on the smooth plains. Only the periods following accretion are directly interpretable within the Borealis region.

Intercrater plains material, which may be a reworked and mixed aggregate of impact and volcanic deposits, was emplaced over a long period that extended past the creation of the Goethe Basin and many smaller basins and craters. The scarps and troughs that trend across intercrater plains material may indicate an early compressional episode that followed even earlier expansion and differentiation of the crust. The size and density of ghost craters that are detectable under the smooth plains material in the interior of the Goethe Basin are indicative of an original basin floor much modified by cratering and emplacement of intercrater materials prior to the emplacement of intermediate and smooth plains materials. This interpretation implies, therefore, that the formation of the Goethe Basin predated or occurred soon after the emplacement of intercrater plains material had begun. The relative similarity in albedo of the Mercurian plains, whether formed of intercrater, intermediate, or smooth plains materials, also suggests a similarity in chemical composition and possibly in mode of emplacement of plains materials. The high crater density of intercrater and intermediate plains materials makes it likely, however, that the original rock types of these two units (whether basalt, impact melt, or impact breccia) were modified considerably by further brecciation following emplacement.

Goethe Basin is considerably older than the Caloris Basin. Emplacement of the smooth plains material of Borealis Planitia during several or many episodes resulted in resurfacing and smoothing of the original material of the Goethe Basin and its surroundings for hundreds of kilometers.

The mercurian surface reached its present configuration several billion years ago (Solomon, 1978). It has been only slightly altered since by impact craters, which are ubiquitously superposed on all other deposits. Generalized summaries of the history of Mercury have been given by Guest and O’Donnell (1977), Davies and others, and Strom.