Ina summit pit-crater: Geology and stratigraphy of a young impact crater and implications for unit substrate characteristics, emplacement sequence and age.

Introduction: Irregular Mare Patches (IMPs) [1] have a rough floor unit (FU) and meniscus-like mounds (MD). Initial Ina CSFD measurements produced ages of ~59 Ma; but Ina is closely associated with an ~3.5 Ga edifice [2]. Could IMP mounds be formed by extrusion of ancient highly vesicular magmatic foams causing impacts to be smaller by ~3X due to formation in compressible magmatic foam [2-3].  Our goal The goal is to distinguish among IMP formation theories [4] by assessing the nature, physical properties, stratigraphic relationships, and ages of Ina units (Fig. 1).  

Major origin/age questions?: 1) Nature of superposed impact craters?: Formation mode, sampling depth, substrate effects on energy partitioning/subsequent degradation, influence on CSFD ages. 2) Predicted and observed regolith thicknesses?: ~59 Ma lava flows impact-generated regolith should be <1 m; MD thicknesses are ~3-5 m, but FL much less; an ~3.5 Ga basalt lava flow on the adjacent volcanic shield should be >5 m. We focus on an extremely fresh ~75m-diameter crater (Fig. 1) on the border between Ina floor and mound units.

Nature of Fresh Crater (FC): The 75 m crater rim crest outline is circular where it intersects the mound to the N-NW, slightly indented to the S-SW, and significantly indented to the SE (Fig. 1a). The crater is characterized by a central floor mound ~1-3 m high, and ranges from MD ~2-15 m deep (max d/D of 0.2, typical of small fresh mare craters) to SW FL, ~2 m deep (d/D 0.0266). The crater floor/lower walls/rim are littered with m-scale boulders. 

Relative and Absolute: Boulders superposed on the main MD, the floor unit and the small SE MD indicates that the crater postdates formation of all these features and initial formation of Ina interior FL and MD units. The median survival time of lunar meter-scale ejecta boulders [5], is ~40-80 Ma, and ~150-300 Ma for ~99% of m-scale blocks. The crater absolute age is clearly in the younger range, placing one of the youngest craters in Ina close to, or greater than the ~59 Ma AMA [1].

Stratigraphy of Target Substrate: The main mound is ~10-13 m above the floor, sloping down to the E. An apparent boundary between the mound and underlying more coherent FL material is seen along the lower crater wall where boulders are exceptionally abundant. Extending adjacent floor unit topographic level laterally into the main mound, we interpret the FL unit to stratigraphically underlie the MD unit.

Characteristics of Ejecta Deposit of the Young Crater: Pre-impact target reconstruction suggests the impact point was centered on the NW MD-FL unit boundary; expanding transient cavity intersected the main MD to NW, the FL to SW and the small MD to SE, offering insights into substrate properties, regolith thicknesses/post-impact degradation. FC shows little evidence of typical fresh young maria craters ejecta deposits.

Physical Properties of Impacted Substrate: Abundance of crater rim/lower interior boulders, presence of central mound suggest that the crater excavated down into a coherent substrate [13] at 0-13 m depth across the sloping mound surface: this value exceeds both regolith thickness model estimates. The nature of the major mound surface and upper crater wall, particularly along the major mound (NNW) wall, suggest a much more incoherent, regolith-like substrate ranging up to ~13 m thick, but retarded excavation on SW FL unit (d/D = 0.0266).

Post-Formation Modification of Main Mound and Crater: A similar-sized degraded crater is observed on the main MD just W of the FC. Its shallow depth (~2 m) and lack of circum-rim boulders suggest that it is >several 100 Ma [5]. Initial analysis of the small SE MD (Fig 1a) appears to indicate that it embays the FC to its NW, indenting its rim circularity, making its formation stratigraphically younger than FC, despite its clearly higher crater density/scattered superposed surface FC boulders. Detailed examination of opposite-side imaging/DTMs, however, reveals a convex-outward topographic mound indentation consistent with FC circularity, exposed boulders along its base, and evidence for mass-wasting of SW mound material down into the FC.

Tentative Conclusions About Ina Age/Mode of Origin: There are multiple contradictions in implied ages of the FL/MD units/stratigraphic relationships: 1) FL regolith thickness (thin; <1 m), optical immaturity suggest very young age, while mound regolith thickness (>5 m) and optical maturity suggest very ancient age: yet the stratigraphic relationships indicate the MD overlie and are stratigraphically younger than the FL, and CSFD AMAs on both units indicate an extremely young age (~58 Ma). 2) MD appears composed of friable regolith-like material throughout its 0-13 m thickness. 3) Both unit fresh craters lack typical distinctive fresh mare crater ejecta. 4) Distribution of fresh crater boulders suggest a young age; possibly as old as several hundred Ma [5], clearly >58 Ma [1]. 5) Extremely degraded mound crater NW of FC much too degraded to have formed <58 Ma ago [5]; lack of surrounding boulders suggest more ancient age. 6) Extremely shallow d/D of crater SW FL part suggests very retarded excavation relative to MD material. 7) The SE DM misleading stratigraphic relationships suggests that craters at FL-MD boundaries are undercounted.

 

Fig. 1. (Left 3) a) (LROC M119815703); b) (LROC M113921307R);  c) LROC M119815703+NAC DTM). Fig. 2.  (Right 2) a) LROC M119815703 showing locations of NAC DTM profiles. b) NAC DTM profiles.

 

References: 1) Braden et al. (2014) Nat. Geosci. 7, 787. 2) Qiao et al (2017) Geology 45, 455. 3) Wilson & Head (2017) JVGR 335, 113.  4) Qiao et al (2017) MAPS 53, 455.  5) Basilevsky (1976) PLSC7, 1005. 6) Zhang et al, (2021) JGR-P 126, e2021JE006880.