Fantastic contribution OldGeo, The dark shape on the Eastern...

Fantastic contribution OldGeo,

The dark shape on the Eastern margin of the image equally corresponds to a mag-high. Attached below is a description of the Marboo formation and Ruins dolerite which intrude it as per the Yampi mapping report (1993). The relevant rock types in the immediate area.

Marboo Formation (Em)

Sofoulis et al. (1971) and Gellatly and Sofoulis (1973) tentatively correlated the metamorphosed turbidites, which constitute the oldest rocks on YAMPI, with the Olympio Formation of the Halls Creek Group, defined in the Halls Creek Orogen (Dow and Gemuts,  1969). Although the lithologies in both units are similar (i.e. they both consist of metamorphosed turbidites), there is not continuous outcrop between them. Units of the  Halls Creek Group seen to underlie the Olympio Formation in the Halls Creek Orogen are not recognized on YAMPI, nor on CHARNLEY nor on LENNARD RIVER to the east and southeast (Gellatly and Halligan, 1971; Derrick and Playford, 1973; Griffin et al., in press). Therefore, it is not felt appropriate to formally ascribe these metasedimentary rocks to the Olympio Formation of the Halls Creek Group, and they are here named the Marboo Formation (Em, Emu, Emh). The top of the unit is not seen, and no base or basement has been recognized.

The type area of the Marboo Formation is in the vicinity of Marboo Pool on the Townshend River. The unit consists of thinly bedded metamorphosed turbidite rocks that also outcrop in the headwaters of the Little Tarraji River and Mangrove Creek, as well as in the southeastern corner of YAMPI. The formation outcrops in the Cone Bay Inlier.  The rocks are intruded by granitoids that give a Rb-Sr date of c. 1840 Ma (Bennett and Gellatly, 1970; Page, 1976; Page et al., 1984). This is consistent with U-Pb dates on zircons of c. 1880 Ma from Halls Creek Group rocks in the Halls Creek Orogen reported by Page  (1988). An Archaean age has been suggested for the metasedimentary rocks (e.g. Gellatly, 1971) based on a Rb-Sr date of c. 2700 Ma from a pegmatite intruding the Halls Creek Group (Bofinger, 1967). Another pegmatite with the same field relationships gave a Rb-Sr model age of 2250 Ma. Page (1976) concluded that this large discrepancy, together with an isochron age of 1755 Ma from highly radiogenic muscovite from the first pegmatite, made the data very difficult to interpret. The available geochronological evidence pointed  to an Early Proterozoic, rather than an Archaean, age for the Halls Creek Group.

The Marboo Formation originally consisted of interbedded mudstone, siltstone and quartz wacke. It was metamorphosed under greenschist facies conditions, with grade reaching the amphibolite facies near Alexander Creek. Sandstone beds are generally 10-30 cm thick and, before metamorphism, typically consisted of fine- to medium-grained, subangular quartz  clasts in a clay matrix. Detrital tourmaline grains are abundant, with lesser amounts of apatite, allanite, rutile, zircon and Fe-oxides. Locally, thicker beds (up to 1 m) of very coarse sandstone occur. The sandstone typically has a sharp base and grades upwards into siltstone and mudstone. The sedimentary rocks are parallel laminated and display many features consistent with deposition by turbidity currents (Walker, 1984). Outcrop is dominated by units showing the BDE Bouma sequence (Bouma, 1962), and the rocks probably represent the lower, more distal part of a submarine fan. Sole marks on the base of sandstone occur sporadically and, although Sofoulis et al. (1971) reported current bedding in sandstone in the Mondooma area, the direction of sediment transport is not known.


Ruins Dolerite (Edr)

The Ruins Dolerite was defined on LENNARD RIVER (Griffin et al., 1993) and here refers to rocks previously mapped as Woodward Dolerite (Sofoulis et al., 1971; Gellatly and Sofoulis, 1973). The Ruins Dolerite is poorly exposed and consists of steeply dipping  metamorphosed basic sills. They outcrop mainly on the southeast corner of YAMPI but also in the headwaters of the Little Tarraji River, Townshend River and Sandy Creek, and in the Cone Bay Inlier, south of Cone Hill. The sills are indistinctly layered and are up to several hundred metres thick (Sofoulis et al., 1971). They intrude phyllites of the Marboo Formation and have been deformed, although this is not well defined due to the poor outcrop and later faulting which has dismembered the units. The Ruins Dolerite is intruded by granitoid.  

Textures in the dolerite (although obscured by recrystallization to amphibolite) are medium to coarse grained, and even grained to porphyritic. Coarse porphyritic p!agioclase (up to 3 cm long) is distinctive and porphyritic **bro forms layers up to 20 m thick (Sofoulis et  al., 1971). Similar, very coarse glomeroporphyritic **bro outcrops on CHARNLEY. One sharp hornfels contact was identified by Sofoulis et al. (1971) whereas most contacts are defined by chloritic schist. Shears within coarse dolerite contain plagioclase augen, and thin epidote veins are widespread.