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Abstract:
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A boulder bed unit in the upper Haymond Formation (Pennsylvanian ) , generally believed to be olistostromes , is exposed in the eastern Marathon Basin , west Texas . Two localities of this unit (Housetop Mountain and Clark Butte ) contain clasts derived from several formations found within the basin , as well as exotic Devonian metamorphic and volcanic rocks . This report describes a third previously unstudied site (Martin Ranch locality ) that contains clasts of exotic Middle Cambrian shelf limestones . These limestones provide a key to the Early Paleozoic history of the Marathon region . The boulder beds lie in the upper part of the Haymond Formation . At the Martin Ranch locality they form a zone that is traceable for 6 .6 km along strike and is up to 230 m thick . These boulder beds contain interbedded units of massive , unstratified , pebble - to boulder -bearing mudstone , thickly bedded , massive sandstone , lenses of pebbly sandstone , and deformed flysch beds . About 80 percent of the clasts found in the boulder beds at Martin Ranch are chert derived from several basin formations . Unique displaced slabs of bedded chert pebble conglomerate comprise about 10 percent of the clasts . Theses conglomerates were probably derived from upper fan -channel deposits within the lower Haymond Formation . Pennsylvanian limestone clasts redeposited from the basin facies of the Dimple Formation and clasts of exotic , late Middle Cambrian limestones each comprise about 5 percent of the clasts . These Cambrian limestones , older than any formation in the Marathon Basin , contain a fauna characteristic of the seaward edge of the cratonic carbonate shelf . The presence of the Cambrian clasts constrains the location of the North American shelf edge during the Cambrian , placing it at least 120 km southeast of the present day Marathon Basin . Both the Martin Ranch and Housetop Mountain boulder beds are composed mainly of clast -bearing , matrix -supported mudstone which have pebbly sandstone , massive sandstone , and flysch beds interstratified with the mudstone and represent periodic deposition of debris flows , slumps , slides , and turbidites interspersed with normal basin deposition of flysch facies rocks . However , different clast types are found at the two localities . The Martin Ranch locality has clasts of Cambrian limestone and chert pebble conglomerate , the latter up to 90 m in length , that are absent at the other localities . Exotic Pennsylvanian limestone clasts and exotic Devonian metamorphic and volcanic rocks , common at Housetop Mountain , are rare or missing at Martin Ranch . The Clark Butte locality is unique because it lacks the mudstone which dominates the other two localities . Instead , the matrix is composed of a pebbly sandstone and conglomerate associated with thick sandstone beds . The boulder beds at this locality may represent upper fan channels and channel -lag deposits . The turbidites and olistostromes resulted from recycling of the southern edge of the tectonic basin as the advancing Ouachita thrusts uplifted the pre Haymond strata . Most of the clasts were from older basin formations exposed by these faults ; however one of these thrusts also uplifted slivers of exotic Middle Cambrian limestone . Earthquakes probably triggered slumps and rock falls off the fault scarps . As the boulders travelled downslope plowing through the slope sediments , they accumulated more material . This combination of slide debris and slope mud turned the slumps and slides into debris flows . Between episodes of debris flows and turbidity currents , normal basin deposition of thinly bedded turbiditic sandstone and pelagic shale occurred . |