December 11, 2013

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Stratigraphy 201

image by Damian Peach

This is a southern continuation of an LPOD from a few weeks ago, getting down into the southwest corner of the Tranquillitatis Basin. This is a not too complicated place for another lesson in stratigraphy, the positioning of events into a time sequence. But this is a little harder, so it is Stratigraphy 201, like the second course in an American university. Stratigraphy is anchored by its extremes, the oldest and the youngest features/events. The youngest always has a number of candidates because most sharp small craters are young, for example, the 7 km wide Arago B near top right. But there are also many secondary craters here as well, mostly from Theophilus about 400-500 km to the south. It has recently been proposed that Theophilus is 3.0 b.y. old, so it and its secondaries would not be considered young. But Dionysius with its light and dark rays presumably is less than 1 b.y. old, definitely young by lunar standards. At the other extreme, the oldest landscape is presumable the highlands that the Tranquillitatis Basin excavated into, but it is not clear if any of that old material is visible. Certainly at upper left the striated smearing suggests that most of the older surface is covered by pasty ejecta from Imbrium, from about 3.8 b.y. ago. Features older than that probably include the pieces of a crater rim that arcs south from Sabine to Schmidt. This, and some smaller nearby ruined craters, were pelted by Imbrium ejecta, and flooded by later Tranquillitatis lavas, about 3.7 b.y. old. The glacies - ejecta building an elevated area around a crater rim - of Sabine and Ritter are embayed by mare lavas, so they formed on the basin floor before the lavas arrived. But both craters also bury rilles - the Hypatia Rilles and the Ritter Rilles - which formed after the lava solidified as extension cracks when the center of the maria subsided. The craters probably formed after some lavas had been emplaced, cooled and cracked, but before the last lavas covered the surface. Ritter (and perhaps Sabine) is a floor-fractured crater, an impact crater which is modified by rising magma that gets trapped under the crater, uplifting its floor. More evidence that the craters were about coeval with the lavas. The rim of Ritter is sharper than that of Sabine, suggesting that Ritter is somewhat younger. Finally, the apparently unnamed sinuous rille northeast of Ritter formed when the lava was flowing so it is the same ages as the youngest local lavas. Stratigraphers always present their results as a column so here goes with oldest at the bottom and youngest on top:

Dionysius, Schmidt, Arago B and other smaller, bright craters <1 b.y. old
Theophilis secondaries 3.0 b.y. old
Tranquillitatis lavas & rilles ~3.7 b.y. old
Ritter ~3.7 b.y. old
Sabine ~3.7 b.y. old
Imbrium ejecta ~3.8 b.y. old
pre-Imbrium ruined craters >3.8 b.y. old
Tranquillitatis Basin >3.8 b.y. old

Chuck Wood

Technical Details
Sept 24, 2013. C14.

Related Links
21st Century Atlas chart 12.
Damian's website

Yesterday's LPOD: Just an Artistic View

Tomorrow's LPOD: X Marks the Spot


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