Deposition and diagenesis of Landslip Hill silcrete, Gore Lignite Measures (Miocene), New Zealand

Abstract

Silcreteexposed at Landslip Hill forms part of the Early Miocene Gore Lignite Measures. The silcrete, well known for its diverse fossil megaflora of leaves and cones, is up to 8 m thick and consists of silica-cemented alluvial channel and channel-margin deposits. Five sedimentary facies are differentiated: quartz pebble conglomerate; trough cross-stratified coarse grained sandstone; planar laminated fine and medium grained sandstone; mottled sandstone pervaded by root structures; and mud-clast conglomerate. Measurements of 113 horizontal log moulds show a preferred north-south alignment, parallel with the north-southpaleocurrent direction indicated by three trough cross-stratification axes.

Field and petrological observations indicate cementation started soon after deposition. Secondary silica consists of spherulitic and granular chalcedony, microcrystalline quartz, and syntaxial quartz overgrowths. Although sandstone in places shows intergranular “pressure solution” textures, the plant fossils are preserved as relatively uncompressedmoulds.

Keywords:

• Landslip Hill beds
• silcrete
• quartz sandstone
• orthoquartzite
• shallow burial
• diagenesis
• silica cement
• paleosols
• fluvial facies
• paleocurrents
• Gore Lignite Measures (Fonnation)
• Early Miocene