| Summary: | Detailed provenance analysis and glauconite morphology of the Broken River and Iron
CreeklWaipara Formations and other correlatives were conducted at 6 localities: the MandamusDove
River area, Waipara River, Avoca-Iron Creek, Castle Hill Basin, Malvern and Mt Somers.
The basal Broken River Formation is a fluvial boulder conglomerate interbedded with sandstones,
mudstones and coal dated as Haumurian (Late Cretaceous) by pollen. The transgression is marked
by a gradual drowning of the fluvial conglomerates with minor glauconite appearing in the beds
immediately overlying the conglomerates in alllocali ties. The estuarine-marginal marine I lower
shoreface succession of the Broken River Formation contains minor nascent micaceous glauconite.
This increases in the overlying Iron Creek I Waipara Formations to 30-60% glauconite composed of
nascent-micaceous to evolved/mature glauconity types characteristic oflower shoreface I foreshore
to shallow shelf depositional setting. Up section, evolved mature glaucony dominates, in some beds
formed in situ (autochthonous) and in other beds transported from nearby (parautochthonous) to
line foresets. An extremely low sedimentation rate is necessary to form the evolved I mature type of
glauconite. The age of the greensands is Teurian to Whaingaroan (Late Paleocene-Late Eocene).
Overall the glauconite analysis indicates extremely low sedimentation rates with autochthonous I
parautochthonous glaucony formation in nearshore marine settings, possibly even estuary
environments.
Clast counts from the basal conglomerates indicate derivation from local sources such as the
underlying Torlesse greywackes (Pahau and Rakaia Terranes) and/or the Mandamus Igneous
Complex. Sandstone composition indicates the addition of more distal sources. Quartzose
sandstones plot in the interior craton province in QFL plots for both Broken River and Iron
Creek/Waipara Greensand Formations. Sandstone lithics are probably derived from the underlying
Torlesse greywacke. Alkali feldspar dominates over plagioclase indicating a probable plutonic
felsic source. SEM-cathodoluminescence on quartz grains indicates a bimodal metamorphic to
plutonic quartz grains with minor volcanic input. Plutonic grains are identified by healed
microcracks, and are possibly derived from Western Province plutonic suites such as the Karamea
and Separation Point Batholiths. Polycrystalline/dark CL quartz grains indicate a relatively high
grade metamorphic source such as the OtagolHaast Schist while dark CL monocrystalline quartz
grains indicate a low to medium metamorphic grade source such as the Alpine Schist. Volcanic
quartz is zoned with straight extinction and was most likely derived from the Cretaceous Mount
Somers Volcanics Group.
Overall the provenance suggests local derivation of sediments when coarse fluvial deposition
occurred followed by more distal derivation once transported in the nearshore marine setting.
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