Begg, J.G. 1994 Stratigraphy of drillholes JS-1 and JS-2, Judgeford, Pauatahanui, Wellington, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 94/03 40 p.
Abstract: Stratigraphic columns for two drillholes in the Judgeford basin near Pauatahanui, Wellington are presented. The drillholes, JS-1 and JS-2, penetrated 11.9 and 44.5 m of Quaternary sediments respectively. JS-1 was sited within 50 m of a pre-existing drillhole that failed to reach basement at a depth of greater than 60 m (200 feet). The maximum thickness of Quaternary sediments in the basin is thought to be between 80 and 100 m. The sediments dip at low angles to the north. The three drillholes and seismic and gravity data, define a steep basement slope on the northwest side of the basin interpreted to be the buried scarp of the Moonshine/Takapu Fault. A minimum vertical offset of the greywacke erosion surface across the fault during the Quaternary of c. 160 m is indicated. The Moonshine/Takapu Fault, and a north-trending splinter fault, cross the Pauatahanui drainage system near the eastern (upstream) extent of the Pauatahanui gorge. It seems likely that the Moonshine/Takapu Fault system has controlled sedimentation within the Judgeford basin. At drillhole JS-1, basement is sheared and shattered, veined argillite. Overlying Quaternary sediments can be divided into four units: 3 m of poorly sorted gravelly silty sand on the basement surface (11.9 to 8.9 m); overlain by 2.5 m of silty sand (8.9 to 6.4 m); overlain by 4.9 m of sandy gravel (with a thin clayey silty horizon between 3.8 and 3.3 m) (6.4 to 1.5 m); overlain by 1.5 m of gravelly clay (?Holocene). Basement in JS-2 is extremely weathered, jointed, light brown to grey sandstone. Basal Quaternary sediments (44.5 to 43.8 m) are extremely weathered, with clast boundaries in gravel difficult to define. The overlying sequence from 43.8 to c. 17.8 m is alternating clay and silt, sometimes carbonaceous, sand, and gravelly sand. The proportions of gravelly sand and clay/silt/sand are approximately equal, in units up to c. 5 m thick. Between 17.8 and 2.5 m, sediments are dominated by clay and silt, sometimes peaty, interbedded with a few gravelly sand beds up to c. 1.5 m thick. The overlying silty gravel (2.5 to 0 m) are thought to be holocene alluvium. The sequence below the ?Holocene gravel is broadly upward-fining. The base of the Quaternary sequence in JS-2 is c. 20 m below sea level, yet no marine sediments are preserved. It is likely that the basin has been isolated for a long time from the currently marine Pauatahanui Arm of Porirua Harbour by the Pauatahanui gorge. (auth)
