BASALTIC RING STRUCTURES IN THE CHANNELED SCABLAND: IMPLICATIONS FOR MARS

In the Channeled Scabland of east-central Washington State, Pleistocene floods scoured the Columbia Plateau exposing numerous basaltic ring structures (BRSs) in the Roza Member of the Wanapum Basalt Formation, which was emplaced ~15 Ma ago. These BRSs are defined by quasi-circular raised annuli 50-500 m in diameter [1]. They occasionally have small peaks at their centers with surrounding moats, but more often have central depressions. At least two distinct populations of BRSs exist: one around Odessa, WA and another west of Tokio, WA. The structures near Odessa typically have concentric rings made up of discontinuous arcuate ridges. Those near Tokio are generally single-ringed structures with little topography and significant loess infill; they are smaller and less circular than the Odessa population. Previous researchers have hypothesized that BRSs form by either the sagging and foundering of crust over an active lava flow [2] or by the interaction of ground water with lava causing venting, doming, and cracking of the crust [1].

In this study we documented in detail 5 multi-ringed BRSs in the Odessa area. In each case the BRS consists of entablature with columns that plunge steeply away from the center and auto-intruded dikes with larger columns that plunge at a moderate angle toward the center of the ring structure. We also examined a purported cross section of a BRS near Banks Lake [2]. This outcrop shows a funnel-shaped depression in the entablature with auto-intruded dikes that have orientations similar to those described above. On either side of the central depression, the lavas appear to be more finely fractured, suggesting rapid cooling. Based on these findings we hypothesize that the BRSs formed where advancing lava flows were diverted around local concentrations of water. Circular features that are similar in shape, size, and distribution to the Tokio BRSs have been observed in Athabasca Valles, Mars, a region where young (~10 Ma) lava flows have been eroded by even more recent catastrophic floods [3]. The apparent role of water in the formation of the BRSs on Earth has important implications for understanding the spatial and temporal distribution of recent water on Mars.

[1] Hodges C A (1978) GSA Bull., 89, 1281-1289. [2] McKee B and Stradling D (1970) GSA Bull., 81, 2035-2044. [3] Burr D M et al. (2002) Icarus, 159, 53-73.