Shakespeare Quadrangle - Geologic History

Geologic History

The history of the Shakespeare quadrangle as evidenced by materials exposed at the surface begins with the formation of intercrater plains material and of impact craters both older and younger than these plains. Some c1 and c2 craters were superposed on the intercrater plains. The intermediate plains material and lineated plains unit were emplaced over the intercrater plains, as were most craters of c3 age. Then followed the major asteroidal impact that produced the Caloris Basin and the emplacement of rocks of the Caloris Group around the basin. Comparison of crater populations on surfaces older and younger than Caloris suggests that at the time of the Caloris impact, the population of craters smaller than 30 km in diameter was eradicated from the pre-Caloris terrain (Guest and Gault, 1976). Gault and others (1976) suggested that the smaller craters were destroyed by the Caloris event and by other basin-forming events elsewhere on the planet at about the same time.

The smooth plains material was then emplaced. Some c3 craters were formed after the Caloris event and after some of the smooth plains were formed. Superposed on the smooth plains unit and on all older deposits were craters of c4 age, inside which was emplaced the very smooth plains material (unit pvs). Analogy with the Moon suggests that most of the recorded events in the history of Mercury occurred during the first 1.5 b.y. of the planet’s life; the oldest major rock units in this quadrangle are probably at least 2 to 3 b.y. old. The geologic history of Mercury has been summarized by Guest and O’Donnell (1977), Davies and others, and Strom.