Tuesday, April 24, 2012

Fieldwork in North Otago

 A couple weeks ago Ewan invited fellow student Cheng-Hsiu Tsai and myself along on some fieldwork in North Otago. We drove up the coast on highway 1 (sounds eerily familiar to my usual routine in California!), while Ewan gave us a geological narration of the drive. For the uninitiated - New Zealand has historically been a source of scattered Oligocene and Neogene fossil cetaceans (and other marine vertebrates), and many early discoveries include the baleen whales Mauicetus parki, "Mauicetus" lophocephalus, "Mauicetus" waitakiensis, and "Mauicetus" brevicollis, the early odontocete Notocetus marplesi, the possible archaeocete Kekenodon onomata, and fossil penguins, sharks, etc. However, it was not until the 1980's when Ewan and his dedicated preparator Andrew Grebneff received a National Geographic grant to excavate more fossils - and since, there has been an explosion in the volume of Oligocene marine vertebrates (particularly cetaceans) from the South Island of New Zealand. Many of these localities are in North Otago and South Canterbury, but I won't give out any more detailed information due to the sensitivity of the localities.
We arrived at this first locality - a lime quarry - to meet up with 3rd year student (and part time fossil preparator) Nichole Moerhouse, who was collecting data for a research project on the depositional environment of the Kokoamu Greensand and the Otekaike Limestone. The Kokoamu Greensand is early Late Oligocene in age (~30-26 Ma), and is a relatively thin richly glauconitic sandstone overlying a temporally significant unconformity, which is angular in places. It gradually transitions to the latest Oligocene-earliest Miocene Otekaike Limestone (~25 Ma), which can be 'sandy' in places, and basally contains glauconite. The Kokoamu-Otekaike section represents an overall shallowing - glauconite can only form in relatively deep environments (at least deeper than 'middle' shelf) during very slow sedimentation), and a gradual transition from slow offshore glauconite-rich deposition to inner shelf/shoreface calcareous deposition with abundantly preserved invertebrates. For those less familiar with stratigraphy and sedimentology - this is a fairly straightforward and commonly encountered type of depositional 'sequence' in marine strata, and generally represents an initial deepening of the shelf (possibly due to the continental shelf being down-dropped due to tectonic subsidence, or an increase in sea level) and subsequent filling of the basin and sediment marching out onto the shelf from the shoreline. Understanding concepts like this is crucial to paleontology, as these processes are going to affect the preservation, abundance, and three dimensional distribution of fossils within a body of rock.

Here, Tsai climbs up an exposure of the Otekaike Limestone to look for cetaceans. This is the only photo my camera took that day - it's particularly finicky and doesn't like cheap batteries, unfortunately for my wallet.

Nicole and I looking at a fossil dolphin in the Otekaike Limestone - it's below Tsai's feet, who took the picture. Ewan had spotted this years before, and sent Tsai and I up to locate it. It was exposed when the original road bed to the quarry was dug out (what we're standing in), and in the time since, a lower road was dug (the trench can be seen to the upper right).

 A fossil baleen whale mandible! Unfortunately it was not in great shape, but there was a thin flat bone above it - which could be a maxilla, the large flat bone in the snout of a baleen whale. It may be worth excavating in the future. We found nothing else of note at this locality. We visited two other spots - at one of which I spotted what most likely is a partial jawbone from a small dolphin. At the third locality for the day, we walked around and found a few cetacean bone fragments, part of a penguin vertebra, a penguin coracoid, a swordfish vertebra, and part of a large shark vertebra. Ewan also found a partial cetacean earbone, part of which had been scraped away by heavy machinery. He went and fetched his special chainsaw - and after 45 seconds of work, had a block of sandstone with the earbone inside. I am extremely interested in bringing this excavation method back to the states, as it reduces the time spent excavating by 75% or so.

Next up: pictures from a hike into native New Zealand forest.

No comments: