New Zealand turbidite field trip – day 1
Today we went to a nice outcrop along the west Taranaki coast to look at Miocene turbidites. These deep-marine sandstones were deposited in a retro-arc setting during the mid to late Miocene – more on that in another post. The Taranaki basin is also a very productive basin in terms of hydrocarbons, with more than 6 trillion cubic feet of gas produced. A great website for more information is the GNS website. Most of the production comes from older Eocene shoreface deposits, but a little bit does come from the Miocene turbidites, which are obviously present in the subsurface, but also outcrop along the coast. These coastal exposures are washed by the large tidal range (~4 m) and so there is virtually no weathering. This makes it easy to see exquisite detail of sedimentary structures. An example of this is below. Make the jump for the interpretation.
From the black beach sand upwards, this bed is planar laminated (a Bouma Tb division), implying Froude supercritical flow. About halfway up the photo there is a change into rippled lamination (a Bouma Tc division), indicating a change to subcritical flow. These ripples are climbing ripples, indicating high sedimentation rates (see this paper by Jobe et al 😉 for details), and about 5 cm above the pen in the left side of the photo, the top of the bed is indicated by the massive grey mudstone. Note that the mudstone drapes over the tops of the ripples, preserving their morphology.
Day 2 will be a visit to some slightly older turbidites with much different facies – see you tomorrow!