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The Lemhi arch was a northwest-trending landmass that controlled marine depositional patterns in southwest Montana and east-central Idaho during the late Proterozoic and early Paleozoic, and provided a source for early Paleozoic clastic sediments. The arch persisted as a landmass until Late Devonian, when it was finally covered by marine sediments.
The arch first formed in the late middle Proterozoic, when middle Proterozoic miogeoclinal sedimentary rocks, the Lemhi Group and Swauger and Lawson Creek Formations were arched into an elongate dome. It was deeply eroded in late Proterozoic, and as much as 5,000 m (16,400 ft) of clastic rocks were stripped away. The eroded edges of the middle Proterozoic rocks on the west flank of the arch were partly covered in late Proterozoic(?) and Early Cambrian by the onlapping Wilbert Formation and Tyler Peak Formation of McCandless, but sedimentation apparently did not continue into the later Cambrian. On the east flank of the arch, marine sedimentation began with deposition of the Middle Cambrian Flathead Formation, and continued through the Late Cambrian, leaving a westward-thinning wedge o marine rocks against this flank.
During the Ordovician and Silurian, the east flank of the arch was emergent. The west flank was partly submerged in the Early Ordovician, and the onlapping nearshore clastic and carbonate rocks of the Summerhouse Formation were deposited. These rocks are successively overlain by the eastward-thinning marine rocks of the Kinnikinic Quartzite (Middle Ordovician) and the Saturday Mountain Formation (later Ordovician and Silurian?). The west flank of the arch was briefly exposed to erosion after deposition of the Saturday Mountain Formation, but was again partly submerged in Middle and Late Silurian, when the eastward-thinning Laketown Dolomite was deposited.
Both flanks of the arch were exposed in Early Devonian, but in Middle Devonian, deposition was renewed on the west flank as fresh- and brackish-water sandstone was deposited in channels cut deeply in the Ordovician rocks. Later Middle and Upper Devonian sandstone, algal dolomite, and sedimentary carbonate breccia indicate eastward, onlapping deposition in a nearshore environment, and these are succeeded by Upper Devonian marine dolomite and limestone; all of these rocks above the channel sandstone are included in the Jefferson Formation. The east flank of the arch was exposed through much of the Devonian, but late in the period a thin sequence of marine carbonate rocks of the Jefferson Formation was deposited across the top of the arch, and marine sedimentation in this region was cont nuous from the miogeocline far onto the craton for the first time since uplift of the cratonic region early in the Middle Proterozoic.
The Lemhi arch continued to influence marine deposition even after it was submerged, separating the region of shelf deposition in southwest Montana and east-central Idaho from the region of miogeoclinal deposition in central Idaho. The arch apparently was a landmass again through much of the Mesozoic; it was overridden by the Medicine Lodge thrust plate, which is composed of miogeoclinal sedimentary rocks, in late Early and Late Cretaceous.
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