Abstract

The ca. 1260 Ma Castner Marble is composed of six metamorphosed lithologies: stromatolitic limestone, cryptalgalaminite, massive limestone, hornfels (mudstone and tuff), rhythmite, and flat-pebble conglomerate. On the basis of the associations of these lithologies, the Castner Marble can be divided into three distinct intervals: a lower section containing stromatolitic and cryptalgalaminite units, a middle section with thick hornfels and massive limestone beds, and an upper section of rhythmites (interlaminated to thinly interbedded carbonate and hornfels) with interbedded flat-pebble conglomerate. The Castner succession was deposited during a transgressive event on a low-energy carbonate ramp. The contact between the Castner Marble and the overlying basaltic Mundy Breccia marks a catastrophic event or series of events, in which rhythmite units were disrupted and/or displaced along arcuate detachment surfaces and redeposited as lenses of rhythmite megabreccia unconformably overlying intact rhythmite sequences. These megabreccia lenses consist of large, locally deformed rhythmite blocks in a matrix of muddy flat-pebble conglomerate. One megabreccia lens contains intervals bearing irregular-shaped basalt clasts possible hyaloclastite layers, and pod-like basalt intrusions. This polymictic lens may have been produced by submarine syn-eruptive mixing of basalt and sediment. Local basalt intrusions may be part of the feeder system for this material and also the overlying basaltic Mundy Breccia. The Mundy Breccia covered the irregular topography created by the megabreccia lenses, locally loading and deforming the megabreccia into flame-like structures. The polymictic (carbonate and basalt) nature of one lens and presence of load features suggests that little time elapsed between deposition of the Castner Marble and the Mundy Breccia, and therefore that the Mundy Breccia was most likely deposited in a submarine setting.

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