Table 11. Reference section of Guyet Formation near the mouth of the first west-draining tributary to Alex Allan Creek (location; 53°08'28”N, 121°30'20"'W). Unit Lithology Thickness in metres Unit Above base 11 10 co [oe) ~ lop) on > w ine} peer matrix, pebble clasts of grey. to olive chert and cherty pelites, and minor light grey siltite, black pelite and sandstone, pebbles are angular MopeuOMOURUCG fic. 66s on Quartzite, grey, coarse grained ... Conglomerate, black shale matrix, pebble and cobble clasts of grey green chert, quartzite and green- stone and are subrounded ....... Greenstone, flow?, poorly exposed and may be a large boulder ...... Conglomerate, green shale matrix, 40% pebble clasts of green cherty eeailite, anQWidh< <6 Conglomerate, quartz sand matrix, 20% pebble clasts of light grey pelite Conglomerate, matrix grading upward from quartz chert sand to black shale, 40% pebble and cob- ble clasts of chert, calcareous greenstone, dark grey pelite, light grey siltite and light green diabase, clasts are subrounded to rounded . Quartzite, grey, coarse grained, has an erosional upper contact ....... Conglomerate, quartz, chert sand matrix, pebbles of grey pelite, grey and green cherts, light grey siltite and minor cobbles of calcareous amygdaloidal greenstone, grades into the overlying unit .......... Conglomerate, black shale matrix, 35% pebbles and cobbles of cal- Careous amygdaloidal greenstone, grey and green chert and cherty argillite, black pelite, light tan quartzite and green and brown silt- stone, clasts are subrounded ..... AEQVIERGO Mretece a ee as ee ee es Quartzite, brownish grey moderately coarse grained Conglomerate, quartz, chert sand matrix, pebbles and cobbles of cal- careous greenstone, dark grey and grey cherts, grey pelites, light grey moderately crystalline limestone and light brown siltstone ............ Base unseen 30 69.5 2 39.5 5 37.5 1 32.5 3 aos pees 26.5 i 24 be) 1 4 14.5 3 13.5 10.5 10 10 underlying the area south of Mount Tinsdale to Black Stuart Mountain does not have the volcanic cobbles and boulders and only rarely has granules or pebbles of greenstone. Muddy conglomerate of the Guyet Formation is found interbedded with black slate of the Black Stuart Formation on Black Stuart Mountain and on the ridge east of there. It is the same as that found in the Tinsdale- Cunningham Creek area north of Cariboo River. A dark grey greywacke and shale sequence with 10 to 75 cm beds overlies the muddy conglomerate on Mount Guyet. The contact with the underlying conglomerate is gradational over 3 m with interbedded conglomerate and greywacke. The greywacke is normally graded and be- comes progressively finer upsection until it is interbedded with dark grey to black slate. The top of the sequence is not seen in this locality but is capped by Greenberry Formation limestone eastward along the ridge from Mount Guyet. Sutherland Brown (1957, p. 34) described greywackes from the Guyet Formation conglomerate which have an average feldspar content of 12%. The greywacke he described is in part believed to include muddy conglom- erate and finer equivalents but the high feldspar content could not be confirmed. Greywacke from the southern slopes of Mount Tinsdale does contain comparable amounts of feldspar to those reported by Sutherland Brown and may have been included in his report. These greywackes, however, overlie Greenberry Formation limestone and are therefore younger than the Guyet Formation conglomerate. On the west bank of Alex Allan Creek opposite the first west-flowing tributary there are outcrops of a feldspar, quartz, and quartzite granule and pebble conglomerate. These grey to dark grey rocks are well graded indicating inverted stratigraphy, if the grad- ing is normal. From dark slate the rocks coarsen upsec- tion. The mineralogy of these rocks is unlike that of the Guyet Formation conglomerate previously described, and is more akin to rocks ascribed to the Snowshoe Group. Uglow (Johnston and Uglow, 1926) and Sutherland Brown (1957, 1963) proposed that the clasts of the Guyet Formation conglomerate were derived from underlying Cariboo Group rocks. Campbell et al. (1973) disagreed. The clasts of the Guyet Formation conglomerate cannot be principally from the Cariboo or Snowshoe groups because those groups do not include primary chert, cherty argillite, tan siltstone and volcanic rocks. The chert and cherty argillite and some of the black pelite clasts of the Guyet Formation conglomerate contain recrystallized radiolarians (Fig. 20). These clasts must therefore be Paleozoic and cannot have been derived from non- radiolarian-bearing Hadrynian rocks. Sutherland Brown (1957) advocated chertification of Cariboo Group rock fragments to account for the chert within the Guyet For- mation conglomerate, but this is unlikely considering the presence of radiolarians in some of the chert clasts. The major erosional unconformity is at the base of the Black Stuart Group and is of Early Ordovician age, therefore 31