A PALEOENVIRONMENTAL STUDY OF SUBSURFACE QUATERNARY SEDIMENTS AT WAINUIOMATA, WELLINGTON, NEW-ZEALAND, AND TECTONIC IMPLICATIONS

A stratigraphic drillhole (WS-1) sited on the floor of the Wainuiomata Valley near Lower Hutt, revealed a 61.6 m thick Quaternary sequence o verlying Torlesse Supergroup greywacke sandstone and argillite. The Qu aternary sediments consist of three sequences separated by disconformi ties. The lower sequence, 10.7 m thick (61.6-50.9 m), consists of fluv ial sediments of probable early Quaternary age. The middle sequence, a bout 48.3 m in thickness (50.9-c. 2.6 m), spans most of the Last Glaci ation, Fluvial/overbank (50.9-42.0 m), floodplain/swamp (42.0-34.5 m), and fluvial (34.5-31.3 m) sediments overlie the disconformity at 50.9 m. Conformably overlying these sediments are swamp and lacustrine dep osits between 31.3 and 4.1 m. Diatoms and algal spores and coenobia sh ow the existence of an extensive lake during much of this sequence, fr om 25.6 to 4.0 m. At the peak of its development, at a drillhole depth of c. 23 m, the lake was >10 m deep and had a high algal biomass. Kaw akawa Tephra (22 600 yr B.P.) occurs near the top of die middle sequen ce at 4.1 m depth. The upper sequence, of Holocene-Recent age, is <2.6 m thick, including 0.6 m of fill. The existence of sediments of the l ower sequence of at least Castlecliffian age (early Pleistocene) uncon formably overlying basement greywacke provides a minimum age for the K -surface in the area. The Last Glacial sediments show evidence of pond ing, which may be a result of one factor or a combination of the follo wing: ponding behind an aggradational terrace of the Wainuiomata River ; landslide blockage of the Black Stream drainage near its confluence with Wainuiomata River; and tectonic deformation. Progressive tectonic deformation since about the start of the Last Glaciation is considere d the most likely dominant factor. This has resulted in the elevation of greywacke basement near the junction of Black Stream and Wainuiomat a River. The nature of the deformation-faulting, tilting, and/or foldi ng-has not been established.