Abstract

The 1.1-b.y.-old Cardenas Lavas were deposited in shallow, hypersaline water, probably under tidal-flat conditions, and in an epicratonic basin that subsided without tilting at about the same rate as the lavas built up. Late in Cardenas time the lavas were deposited subaerially in at least some places. Subsequently, the lavas were probably tilted gently to the northeast, then eroded and buried by the Nankoweap Formation, which was deposited in shallow sea water. Most of the lavas exposed today issued from nearby vents. The pre-Nankoweap tilting is in the same direction as a later and more intense pre-Tapeats tilting, suggesting reactivation of existing structures. If the earlier tilting occurred shortly after deposition of the lavas, the reactivation occurred following a very long period of relative quiescence (300+ m.y.). If the pre-Nankoweap erosion interval was long, the tilting events would have been relatively close in time, the erosion interval would have been a major hiatus in the Grand Canyon sequence, and the Nankoweap Formation and Chuar Group would be much younger than the Unkar Group, of which the Cardenas Lavas are part.

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