NEW ENIGMAS IN THE PATTERNS OF SILICIFICATION FROM THE CAMBRIAN WEEKS FORMATION LAGERSTäTTE

The Cambrian Weeks Formation in the House Range of Utah is famous for its well-preserved articulated trilobites. However, hidden in its interbedded shales and lime mudstones is a fauna of silicified trilobites. The silicification is delicate, but innot complete, and therefore most of the complete sclerites recovered represent immature individuals. This may also explains why acetic acid apparently works better than the stronger chloric acid for the recovery of well-preserved trilobite remains. The trilobites recovered via acid digestion are also known from the articulated fauna, but with a different relative abundance. In one unique block, there are silicified sclerites exposed at the shaley parting and unsilicified trilobites within the lime mudstone that readily crack out. Elsewhere, silicified trilobites occur in both the shaley partings and the lime mudstones. Rarely, inarticulate brachiopods have their original calcium phosphate replaced with silica which is very unusual. The silicification is regionally and stratigraphically variable. On the west side of the House Range, the Weeks Formation is too metamorphosed for the recovery of silicified trilobites. On the east side of the House Range, silicified trilobites are recoverable at most sites, but not all. The quantity of silicified material recovered by acid digestion varies from very little to quite abundant (with even minute larvae preserved). We tentatively postulate that some of this variation in the patterns of silicification may be related to an igneous intrusion within the core of the House Range.