43rd Lunar and Planetary Science Conference (2012) 1048.pdf

EJECTA DISTRIBUTION AND RELATIVE AGES OF THE IMBRIUM, SERENITATIS, AND CRISIUM BASINS. W. A. Ambrose1, 1Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924, [email protected].

Introduction: LOLA data reveal an extensive sys- along the east rim of the Serenitatis Basin [11, 12]. The tem of ejecta features associated with the Serenitatis, morphology of the Sculptured Hills and highlands ter- Crisium, and Imbrium Basins (Fig. 1). These ejecta rain to the east superficially suggests an origin as Se- features consist of radial valleys, asymmetric secondary renitatis ejecta. However, the dominant orientation of craters, and crater chains. The distribution, orientation, lineated terrain in this area is azimuth 245 (azimuth 0 and overlapping relationships between these ejecta defined as due north), radially aligned with the Imbri- features provide an improved insight into the sequence um Basin. Moreover, the presence of radially aligned of basin formation. This sequence is confirmed to be ejecta that overlap craters such as Posidonius and Le from oldest to youngest, (1) Serenitatis, (2), Crisium, Monnier which post-date the Serenitatis impact event and (3) Imbrium, consistent with findings from a recent are inconsistent with a Serenitatis origin. The great study of the Sculptured Hills east of Mare Serenitatis distance of ejecta features in the Serenitatis-Crisium [1]. Overlapping relationships between Imbrium and highlands and Sculptured Hills from the center of the Crisium ejecta northwest, north, and northeast of the Imbrium Basin (~1,200 km) may call into question Crisium Basin provide a context for refining previous their origin as Imbrium ejecta. However, Imbrium ejec- interpretations of the age of terrain in areas marginal to ta occur at comparable or greater distances in the these basins [2, 3, 4, 5, 6]. Southern Highlands northwest of the Nectaris Basin [8, 13]. Other Imbrium ejecta north of the Serenitatis- Crisium highlands, 1,000 to 1,200 km from the center of the Imbrium Basin, with orientations ranging from azimuths 260 to 275, overlap Lacus Somniorum and highlands terrain near Franklin and Cepheus (Fig. 2). Serenitatis ejecta. Four radial valleys in the Sereni- tatis-Crisium highlands do not share the dominant alignment (azimuth 245) of Imbrium ejecta and are interpreted to be Serenitatis ejecta (Fig. 2). They occur 200 to 220 km east and southeast of Posidonius, with azimuths ranging from 270 to 285. Although radial valleys in the Taurus-Littrow region have orientations Figure 1. LOLA topography and ejecta features asso- consistent with a Serenitatis origin, they are also sub- ciated with the Serenitatis Basin (S), Crisium Basin parallel to Imbrium ejecta west of the Crisium Basin (C), and the east and southeast margins of the Imbri- that have an average azimuth of 240 and therefore may um Basin (Im). Ejecta are denoted by arrows. Orienta- instead be of Imbrium origin. In contrast, radial valleys tion of arrow indicates direction of ejecta formation. (average azimuth 215) in Montes Haemus on the south Arrow colors: red= radial valleys; yellow = asymmet- rim of the Serenitatis Basin, west of Plinius, are clearly ric secondary craters; orange = crater chains. Details of Serenitatis origin, having an oblique orientation rela- of the Serenitatis Basin and Serenitatis Highlands are tive to Imbrium ejecta to the southwest (Fig. 2). shown in Fig. 2. Details of the Crisium Basin are Crisium Basin and Imbrium Ejecta: Crisium is shown in Fig. 3. an elongate Nectarian basin formed by a low-angle impact [12, 13, 14, 15]. It has two well-developed ejec- Serenitatis-Crisium Highlands: Highlands east ta fields distributed northeast and southeast of the basin of Mare Serenitatis and northwest of Mare Crisium are (Fig. 3), reflecting an oblique impact from the west [9]. composed of hills and lineated terrain that contain ~20 West and Southwest Margins. The majority of radial valleys and ~10 shallow-floored, asymmetric ejecta on the west and southwest margins of the secondary craters (Fig. 2). Asymmetric secondary cra- Crisium Basin were previously interpreted from Lunar ters also occur in peripheral areas of the Nectaris, Orbiter data as Imbrian terra and plains material [5, 6]. Crisium, and Humorum Basins [7, 8, 9, 10]. Serenita- Examples include Tisserand A (~20.5ºN, 49.6ºE), a tis-Crisium highlands also include the Sculptured Hills, shallow-floored asymmetric crater that impinges on the terrain composing knobby highlands material and light west margin of the Crisium Basin, interpreted in this plains units in the vicinity of the Taurus Mountains and study as an Imbrium secondary crater. Other examples the Apollo 17 landing site, and extending northward 43rd Lunar and Planetary Science Conference (2012) 1048.pdf

Figure 3. LOLA topography and ejecta features asso- Figure 2. LOLA topography and ejecta features asso- ciated with the Crisium Basin (C). Ejecta are denoted ciated with the Serenitatis Basin (S) and the Serenita- by arrows. Orientation of arrow indicates direction of

tis-Crisium Highlands. Ejecta are denoted by arrows. ejecta formation. Arrow colors: red= radial valleys;

Orientation of arrow indicates direction of ejecta for- yellow = asymmetric secondary craters; orange =

mation. Arrow colors: red= radial valleys; yellow = crater chains. Ejecta provenance is indicated by c

asymmetric secondary craters; orange = crater chains. (Crisium), and u (unknown) adjacent to arrows. Unla-

Ejecta provenance is indicated by s (Serenitatis), c beled arrows indicate Imbrium provenance.

(Crisium), and u (unknown) adjacent to arrows. Unla- beled arrows indicate Imbrium provenance.

include lineated terrain south of Franz (~15.5ºN, References: [1] Spudis P. D. (2011) LPS XLII, Ab- 40.5ºE), northeast of Macrobius (~23ºN, 47ºE), and stract #1365. [2] Wilhelms D. E. (1987) USGS Prof. south-southwest of Tralles A (~25.5ºN, 46.5ºE). How- Paper 1348, 302 p. [3] Casella C. J. and Binder A. B. ever, an unnamed asymmetric secondary crater be- (1972) Geol. Map Cleomedes Quad., tween Macrobius and Macrobius A (19.7-20.2ºN, 41.8- http://www.lpi.usra.edu/resources/mapcatalog/usgs/I70 43.2ºE), previously mapped as Imbrian [5], may be 7/. [4] Grolier M. J. (1974) Geol. Map Geminus Quad., Serenitatis ejecta, being more degraded than other http://www.lpi.usra.edu/resources/mapcatalog/usgs/I84 nearby ejecta. Elongate scoured terrain west of Apollo- 1/. [5] Scott D. H. and Pohn H. A. (1972) Geol. Map nius A (~6-8ºN, 52-55ºE), are likely Imbrium ejecta, Macrobius Quad., owing to being well-preserved and being oriented sub- http://www.lpi.usra.edu/resources/mapcatalog/usgs/I79 parallel to other Imbrium ejecta to the northwest. 9/. [6] Olson A. B. and Wilhelms D. E. (1974) Geol. North and Northeast Margins. An extensive (400- Map Mare Undarum Quad., km) field of southeast-trending ejecta features, inferred http://www.lpi.usra.edu/resources/mapcatalog/usgs/I83 to of Imbrium origin, occurs north of the Crisium Basin 7/. [7] Clark M. (2006) The Lunar Observer, June, 4– (Fig. 3). The presence of both Imbrium and Crisium 10. [8] Ambrose W. A. (2009) LPS XL, Abstract ejecta provides age constraints for features in this re- #1015. [9] Ambrose W. A. (2010) LPS XLI, Abstract gion. For example, superposition of Imbrium ejecta #1061. [10] Ambrose W. A. (2011) LPS XLII, Abstract onto Geminus glacis, not recognized in [4], confirms #1035. [11] Wolfe E. W. et al. (1981) USGS Prof. Pa- Geminus to be post-Imbrian in age. In addition, south- per 1080, 280 p. [12] Head J. W. (1974) Moon 9, 355- east-trending radial valleys ~60 km east of Burckhardt 395. [13] Wood C. A. (2003) The Modern Moon, 209 A (~31ºN, 63ºE) onto terrain mapped as Imbrium p. [14] Spudis P. D. et al. (1997) LPS XXVIII, Abstract plains units [3] suggests this terrain may instead be #1296. [15] Schultz P. H. and Stickle A. M. (2011) Nectarian in age. LPS XLII, Abstract #2611.