PLASMIC FABRIC FROM THE 1450 MA HELENA FORMATION AT HUNGRY HOARSE DAM, MT

The Helena Formation at Hungry Horse Dam is about 2000 m thick. A 120 m thick section was measured at a scale of 1.5 m per inch. The base of the section lies about 500 m above the top of the Empire Fm. Of 63 samples collected, 6 show plasmic fabric (PF). Of the 31 thin sections cut and stained for calcite and iron, 7 contain PF. The base of the section is located at 43^o 92'' N 109^o 01' 25.67'' W. The section is composed of 52% claystone, 43% silty claystone, 1.7% intraformational conglomerate, 1.3% sandstone and 0.6% stromatolites. About 59% of the claystone and silty claystone is composed of 0.5 to 15 cm thick graded beds (aka couplets). The data consists of images of thin sections taken with a Leitz petrographic microscope and a 9 mega-pixel camera giving a resolution of 1 µm. The PF layers occur in silty claystone- mudstone graded beds.

Plasmic fabric is observed by comparing images taken in crossed-polarized light (XPL) oriented parallel to bedding with an image taken in crossed-polarized light, rotated 45^o to bedding (XPL R45). The XPL R45 image transmits more light than the XPL image reflecting the bedding parallel alignment of clay and silt size platy minerals. Layers displaying PF range from 2 to 10 mm thick and comprise up to 90% of a thin section.

Massive texture PF layers display very well sorted, bedding parallel oriented (BPO) grains. Spotted texture PF layers have widely scattered BPO grains up to 35 µm diameter in a massive PF layer. PF layers with thin striae display 1 to 3 grain thick, well sorted BPO grains in massive PF. PF layers with thick striae have well sorted layers about 65 µm thick of large BPO grains in a finer grained PF matrix.

The top of some PF layers show erosion, recording the incohesive nature of PF layer prior to deposition of the next claystone layer. PF layers display BPO grains even where the layers were folded by differential compaction of very high porosity sediment around concretions (aka pods). Also, fluid escape structures cross-cut PF layers. These structures attest to very early PF formation by mechanisms within the depositing current.

PF has been reported in fluid mud deposits off the coast of Louisiana and from the Amazon Delta. This may be the first examples of PF from well indurated Mesoproterozoic rocks.