MYSTERIOUS 'PODS' IN THE MIDDLE SILURIAN SHAWANGUNK FORMATION, MID-HUDSON VALLEY, NEW YORK

Mound-like conical ‘pods' in the upper part of the Middle Silurian Shawangunk Formation, mid-Hudson Valley, New York, were recently discovered in the Ellenville Tongue near Lake Minnewaska, about 45 m above the underlying unconformable contact with the Martinsburg Formation and just below the overlying the High Falls Shale. The lithology is a medium-bedded, medium-grained crossbedded quartzite and conglomerate with quartz pebbles locally up to about 2 cm in diameter. The holes range from .2 to 7 cm across (at the base) and .2 to 6 cm in height. Internally, the ‘pods' are mostly non-recrystallized quartz grains with interstitial mica and possibly clays. In contrast, the areas outside the ‘pods' consist of pressure solution welded quartz grains consistent with the quartzitic nature of the formation. The ‘pods,' aligned on bedding surfaces (N20E; 38NW) and restricted to a very narrow stratigraphic interval of about 170 cm, show no evidence of internal bedding. The ‘pods' may be either: (1) microbial mounds, (2) sponges or, (3) mud balls. They are similar in general outline to some thrombolites in that the internal texture is non-laminated but there is no indication of microbial activity such as clotting. The possibility that they are sponges is somewhat doubtful because the braided stream depositional environment was one of relatively fast-moving fresh water draining mountains to the southeast; in addition, most sponges live in a marine environment. The conical shape does not favor a mud ball origin.