A paleoenvironmental study of subsurface Quaternary sediments at Wainuiomata, Wellington, New Zealand, and tectonic implications

Abstract

A stratigraphic drillhole (WS‐1 ) sited on the floor of the Wainuiomata Valley near Lower Hutt, revealed a 61.6 m thick Quaternary sequence overlying Torlesse Supergroup greywacke sandstone and argillite. The Quaternary sediments consist of three sequences separated by dis‐conformities. The lower sequence, 10.7 m thick (61.6–50.9 m), consists of fluvial sediments of probable early Quaternary age. The middle sequence, about 48.3 m in thickness (50.9‐c. 2.6 m), spans most of the Last Glaciation. 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 deposits between 31.3 and 4.1 m. Diatoms and algal spores and coenobia show the existence of an extensive lake during much of this sequence, from 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. Kawakawa Tephra (22 600 yr B.P.) occurs near the top of the middle sequence 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 lower sequence of at least Castlecliffian age (early Pleistocene) unconformably overlying basement greywacke provides a minimum age for the K‐surface in the area. The Last Glacial sediments show evidence of ponding, which may be a result of one factor or a combination of the following: 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 considered the most likely dominant factor. This has resulted in the elevation of greywacke basement near the junction of Black Stream and Wainuiomata River. The nature of the deformation—faulting, tilting, and/or folding—has not been established.