Investigating Crater Inlet Valley Formation: Field Study at Lonar Crater, India

On planetary bodies, impact craters and fluvial activity interact, and valley incision competes with the topographic, lithologic and structural disruption caused by impacts that frequently occurred in the geologic past. Yet, many terrestrial and martian impact craters were breached by inlet valleys, which supplied (or still supply on Earth) crater interiors with water. Radial and concentric drainage patterns are also observed around craters, suggesting impact-induced structure fundamentally influences incision in these areas.

To gain a greater understanding of fluvial erosion in crater-dominated terrains, and inlet valley formation across crater rims, we will investigate the incision history of the Dhar valley inlet at Lonar Crater, Maharashtra, India. Lonar crater is the best-preserved impact crater in basalt, which formed within the last 100 ka when a bolide impacted the Deccan Traps basalts. At 1.8 km diameter and 135 m deep, it is a simple crater. A small, 5.5 m deep lake resides in the crater interior and is fed by the Dhar inlet to the north east, and groundwater springs in the crater walls. We would use cosmogenic radionuclide dating to investigate the onset and timescales of fluvial erosion that formed the inlet valley, with comparison to the surrounding non-cratered terrains. We plan to measure the accumulation of cosmogenic ³He in pyroxene and olivine to derive in situ exposure ages at different levels in the valley, and also to derive basin-averaged denudation rates from fluvial sediments. Vesicle-fill quartz is also present, so measurement of cosmogenic ¹⁰Be is a possible complement to ³He measurements.

We also plan to complete detailed mapping of the Dhar valley inlet and examine hypotheses relating to Dhar valley inlet formation. Previous authors have posited that the Dhar valley inlet formed as spring activity promoted drainage head erosion across the steep crater rim and/or that gullying concentrated in the north east of the crater due to water supply from higher elevation regions in that direction. We will also investigate whether a prominent fracture in the north east, and sub-vertical cooling fractures that trend NE-SW (an original basaltic flow feature), may have influenced the Dhar valley inlet formation.

Increased constraints on crater inlet valley incision mechanisms, controls, and rates, will help extrapolate our understanding of fluvial erosion to crater-dominated terrains, including key specific sites such as Jezero crater on Mars, and in generalized numerical simulations of cratered landscapes. This work will ultimately help place constraints on the extent, absolute timing, environments and mechanisms required to develop fluvial valleys around and into impact craters.

Field work is expected to be completed in early Spring 2023 and at EGU 2023 we will present preliminary findings from the field and detail our next steps moving forward. This work is possible thanks to funding from the Eugene and Carolyn Shoemaker Impact Crater Research Fund and graduate field work funding from the Jackson School of Geosciences.