DEPOSITIONAL HISTORY OF THE MIDDLE PROTEROZOIC CASTNER MARBLE AND BASAL MUNDY BRECCIA, FRANKLIN MOUNTAINS, WEST TEXAS

The ca. 1260 Ma Castner Marble is composed of six metamorphosed lithol ogies: 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 Ma rble can be divided into three distinct intervals: a lower section con taining stromatolitic and cryptalgalaminite units, a middle section wi th thick hornfels and massive limestone beds, and an upper section of rhythmites (interlaminated to thinly interbedded carbonate and hornfel s) with interbedded flat-pebble conglomerate. The Castner succession w as deposited during a transgressive event on a low-energy carbonate ra mp. 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 detachm ent surfaces and redeposited as lenses of rhythmite megabreccia unconf ormably overlying intact rhythmite sequences. These megabreccia lenses consist of large, locally deformed rhythmite blocks in a matrix of mu ddy 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 pr oduced by submarine syn-eruptive mixing of basalt and sediment. Local basalt intrusions may be part of the feeder system for this material a nd also the overlying basaltic Mundy Breccia. The Mundy Breccia covere d 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 l oad features suggests that little time elapsed between deposition of t he Castner Marble and the Mundy Breccia, and therefore that the Muny B reccia was most likely deposited in a submarine setting.