SEDIMENTATION AND SIGNIFICANCE OF THE NUIA-BEARING UNITS IN THE LOWERMIDDLE ORDOVICIAN ANTELOPE VALLEY LIMESTONE (AVL) - IN CENTRAL NEVADA, US

In central Nevada the Nuia pack-and grainstone lithofacies constitute the lower cliffs of the Lower Middle Ordovician Antelope Valley Limest one (AVL). Tn these pack-and grainstone units, Nuia, a problematic alg a, is the primary kind of particle. The Nuia pack- and grainstone lith ofacies occur in the lower intervals of the AVL and are interpreted to represent the algal subtidal shoal bars. We believe that these shoals may have formed in a N-S extending nearly flat shelf margin where the Nuia subtidal shoal bars left behind a progressively shoaling, restri cted-deep middle shelf. The Nuia meadow, a main sediment-producing nat ural factory, is interpreted to have occurred mainly in an environment ranging from the open seaward-extended flanks to the crests of these subtidal Nuia bars. The energy level of these shoals is interpreted to have ranged from moderately high to high; this magnitude was much hig her than that of the Girvanella-dominated depositional sites. Thus, Gi rvanella-flourished environments were probably located at the protecte d sites of the subtidal Nuia shoals. At these depositional sites, the high-turbulence and -agitation level of the Nuia shoals inhibited the blue-green algae Girvanella to grow and flourish. However, Girvanella and Nuia coexist in other Nuia-bearing lithofacies of mid-and open she lves. It is likely to suggest that Nuia particles may have been transp orted from the Nuia meadow to open seaward and leeward by wave and cur rent activities. On the other hand, in the Nuia-rich oncoidal packston e lithofacies, the multiwall-structured Nuia particles occur as the pr edominant nuclei of Girvanella-constituted oncoids. This suggests that the multiwall-structured Nuia may have preferably flourished in the G irvanella-dominated soft substrate. Therefore, these kinds of Nuia pro bably represent a different Nuia species than those primarily inhabite d in the Nuia shoals. Fine siliciclastic sediments are closely related to the Nuia-bearing lithofacies of open and mid-shelf facies and to t he Nuia shoal bars at the upper Nuia shoal cycles. At these intervals, upward progressively thinning Nuia shoal bars represent the basal com ponent of the shoal cycles and are directly overlain by thicker fine s iliciclastics. We believe that these relations resulted from short dur ational, high-amplitude sea-level rises which induced incipiently drow ned Nuia shoals. Consequently, drowned-Nuia shoal bars allowed fine si liciclastics to advance farther open seaward. The bedforms and texture s of the Nuia pack- and -grainstone lithofacies suggest a storm-and ti de-dominated shoal facies. However, the absence of ooids and oolitic s ediments in the Nuia-dominated shoals can be attributed to (1) a nearl y flat, wide-shelf margin, and (2) a high-sediment production rate of Nuia. Therefore, Nuia shoal units, with indicated bedforms, predominan t sediment type, and with their relations with fine siliciclastics, ca n probably be analogous to the modem Halimeda subtidal sand bars in so uthern Florida (Shinn et al. 1990).