GEOLOGY OF THE - GNS Science

Figure 24 Gently dipping mudstone of the Early Miocene Whakai Formation beside State Highway 35 near Hicks Bay (214f778874)is the prevalent lithology with in the Tolaga Group. The thin , brown-stained sandstone beds define bedding and show that the fracturedappea rance of the mudstone is a surface effect resulting from wetting and drying cycles. A small fault dips toward the person.Photo CN42200119: DL Homergrained, bioclastic limestone interbedded with calcareoussand stones. Large oy ster shell s are common . TheKouetumarae Limestone isan equi valent present locallybetween the lower M ata Ri ver and Ihu ngia River; itconsists of up to 20 m of poorly bedded, fl aggy, bryozoancoquina lim estone and shelly, bryozoan-ri ch sandstone.The limestones are shallow mari ne deposit s, inferred toha ve accumul ated at shelf depths (Kenny 1984b;Mazengarb el al. J99 1a; Field, Uruski el al. 1997).Southwest of Rakauroa and Te Karaka the Talaga Grouprocks fo rm bluffs and spectacular southeast-facin g dipslopes along the northwestel11 margin of Mioceneoutcrop,At Hangaroa River and Mutuera Stream the basalsediments incl ude a thjn conglomerate, cross-bedded,shell y, pebbly sandstone (Joass 1987) and muddyfo ssil iferoussandstone of late Earl y M iocene (Altonian)age (M.ls). The more typical facies are bedded calcareousmudstone, minor sandstone, and iI1lervals of ahernatingwe ll bedded sandstone and mudstone (MI ). Alternatingsandstone and mudstone un its are locally distinguished(Mia). Near Wharekopae, Savella ( 1992) mapped two(informal) unitsof centimetre- to decimetre-bedded, finetomedium-grained sandstone and mudstone, namely hisCocos fl ysch (which includes fossil coconuts; Ballanceel al. 1981) and Rere sandstone. In the vici nity of theMokonuiorangi Range the Cocos fl ysch and overl yingmudstone pinch out westward beneath the overlying Reresandstone,Middle and Late MioceneNear Raukokore Torlesse composite ten'ane basementandrocks of the Eas t Coa st A lloc hthon are overlainun confo rmabl y by Middle Miocene To laga Groupconglomerate, sandstone, mudstone and limestone (l\tOh ;29