A FIRST GEOLOGICAL MAP OF MAKIRA, SOLOMON ISLANDS: STRATIGRAPHY, STRUCTURE AND TECTONIC IMPLICATIONS

This report presents the first ever comprehensive account of the geology of Makira (formerly San Cristobal). Makira is the last large island of the Solomon Islands archipelago to be systematically geologically mapped at a scale of 1: 50, 000. Makira is readily sub divisible into the Makira Basement Complex (MBC) and the unconformably overlying Makira Cover Sequence (MCS). Currently available Ar - Ar whole rock radiometric age data indicates a Cretaceous - Oligocene (c. 98Ma - 34 Ma) for the Makira Basement Complex. The Harigha Sandstone Group, a unit within the Makira Cover Sequence has yielded Mid Miocene - Early Pliocene foraminiferal ages. The Makira Basement Complex forms the great bulk of the volume of Makira (probably c. 90%) of which basaltic lithologies comprise >65%, and locally almost 100% of the sequence. The Makira Basement Complex comprises basalt and dolerite sheets and minor dykes with gabbroic and ultramafic sheets and intrusions (herein termed the Wairahiti Volcanic Group or WVG) with interbedded sedimentary limestones, cherts, sandstones, and basaltic breccias (herein termed the Waihaoru Sedimentary Group or WSG). The Wairahito Volcanic Group comprises a sequence of basalt, dolerite and gabbro sheets and dykes with local ultramafic sills and intrusions. A cumulate thickness is difficult to estimate as there are few regional stratigraphic units which can be used for correlation purposes; however geological cross sections suggest a minimum thickness of 2km. The Waihaoru Sedimentary Group rocks have a wedge or lens like morphology, with a local thickness maximum region which quickly pinches out along strike. Individual beds can attain considerable (tens to >100m) thickness locally. Monolithic basaltic breccias are particularly common within the WSG in the north of Makira. There are no diagnostic stratigraphical units within the basement which crop out on a regional basis, making cross-island correlation very difficult. The Makira Cover Sequence comprises two main rock groups and an uppermost Quaternary - Recent sequence of alluvium, raised reef, beach, and mangrove swamp deposits. The Upper Miocene - Lower Pliocene Harigha Sandstone Group (HSG) comprises a varied sequence of weak - moderately cemented poorly - moderately sorted, soft, and sometimes chalk - like, calcareous sandstones and