Coarse-grained and clast-supported deposits

Pebble to boulder grain size dominated, and/or clast-supported deposits can be an impediment to OSL sampling with a metal tube. For these deposits, the sediment must be collected under dark conditions, sometimes referred to as “ninja” sampling or “under-the-cloak” sampling. 

Coarse rock

“Ninja” sampling is a learned skill, but it is fun!

The sample must be collected at night or under a cloak of darkness (light proof tarp, heavy cloth, heavy sleeping bag, black-out curtains…) anything that will keep all the light out. 

1

Step 1: Find, photograph, describe and asses the outcrop/trench you plan to sample in the light

Think through all the factors that can affect the mixing of sediment (soils, insects, roots) and induce dose rate heterogeneity (try to stay clear of contacts and unconformities or different texture/composition deposits).

IF you need to sample near a contact, make sure you are at least 10 cm away so you don’t accidentally sample that material when in the dark.

2

Step 2: Collecting the DE and DR samples

At night or under the cloak you can use a red headlamp (red bike tail light works well) to clear off the exposure so you get all of the light exposed sediment cleared back (especially sediment that might fall into your sample from above), at least 0.5 m. 

 

DR sample: Collect a fraction of this material for about half of the dose rate sample (1/4 quart-sized baggie).

If the sediments contain very large clasts, make sure you get a representative collection of all grain-sizes – you might need to collect into a slightly larger bag if that is the case. Do not use canvas rock sample bags as fine particles dissipate from the bulk sample.

 

DE sample: Then using a hand trowel or rock hammer, collect the sample for dating into a black garbage bag. You should focus on excavating from a relatively small area, so the dose rate environment is constant (10-20 cm x 10-20 cm area if possible, slightly wider than tall is ok). 

It is not necessary to keep all of the cobbles and gravel in the DE sample, as there are representative proportions of all grain-sizes in the dose rate sample.

This will lighten the volume of sediment collected.  In the end you want something the size of a volleyball in the black garbage bag. 

Coving rock

3

Secure the DE sample in the dark

  • Under dark conditions, twist the bag top and invert the extra bag material over itself so that you are wrapping it in a couple times in black plastic.
  • Do this as many times as the bag size will allow, use duct tape to keep it intact.
  • Wrap the package up in another black plastic garbage bag – twisting and inverting and wrapping it in as many layers as possible – finishing with a solid duct tape outer wrapping.
4

Collect remainder of DR sample, H2O sample, document location

  • Collect another DR sample from the back of the hole you excavated for the De sample and within ~10 cm of the sample site – put all of that into the same DR sample baggie that you started with.
  • Collect a water content sample and measure the depth below the land surface, photograph and make notes on what you actually collected in the dark.
6

Shipping

Send samples to:
USU Luminescence Lab
1770 North Research Parkway, Suite 123
North Logan, UT 84341

All samples should be labeled 'Light-sensitive mineral samples for geologic analysis. All material will be consumed in analysis. No monetary value'.
International samples: Send via international shipping company such as UPS, FedEx, DHL, TNT, do not use local postal service.

Email tracking number to Tammy or Michelle.