Wednesday, July 17, 2013

Friday, July 12, 2013

Regolith and Quarternary Sedimentology: questions to infer paleogeomorphology and paleoclimate

This image was taken along Las Huertas creek, near the village of Placitas.  Note the lack of developed soil horizons: the top layers appear to be unconsolidated colluvial debris.   Perhaps the sandstone layers below the Juniper roots are paleo-sand dunes?  What then might the different colors indicate?

This image and the following were taken along La Jencia creek, deeply incised into Pleistecene and Holocene sediments in the San Lorenzo Spring quandrangle west of Socorro, NM.    The layers exposed along the creek channel show darker clay and/or organic-enriched layers that may have formed from swamps and/or backwaters along paleochannels.
Screenshot of the Quandrangle geological map, with a red dot on La Jencia creek showing the location of the photos.
This exposure reveals an unconformity in the left-center that may be due to in-filling of a paleo channel?  Does this images show an actual unconformity, with deposition, then erosion, then deposition? Or was there continuous deposition? Why is there banding of light and dark material in such regular layers?  How were these layers laid down?  Does the fact that they were deposited indicate an aggrading landscape, perhaps controlled by climate-influenced sediment supply??

This photos shows a close-up of a tiny (5-foot long) layer of darker clay, clearly deposited in a concavity.  Note the coarser sediment deposited below it and the finer sediment above.  How old are these layers?  How do geologists infer the direction of paleoflow?  Why aren't there fossils?

Surface geology maps of the area offer confusing clues to interpreting these buried layers.  The geological map for the quandrangle to the North of San Lorenzo Springs (the Silver Creek quandrangle) shows paleochannel flows on the surface, as well as relictual dunes from some point in the Quarternary.  Why are the paleochannel flows going every which way?  Was this whole valley a closed basin, and if so, would that explain the aggradation, independent of sediment supply?  Why is this stream downcutting so rapidly today?  What are the implications for the future?

Riparian Restoration Quandary

Plants along rivers face a basic quandary: the closer to the channel they grow, the more water they have.  But, closer to the channel, there is more disturbance from flooding.

  • Is this a healthy/unhealthy stream bank?
  • Is it in need of restoration?

It has:
  • invasive species
  • sparse vegetation
  • obviously eroding banks....

It might be important to ask: What is bankfull here? 

It might also be important to know that this is actually a dry wash, photographed after the first rain in 9 looks grazed, and it is grazed.  By a herd of native Elk, forced out of the uplands by the worst drought in 50 years.

We have to be careful to set our ecological expectations to the history of natural disturbance and the reality of a changing climate.

Thursday, July 11, 2013

Gaillardia grandiflora

Photomicrograph of an Asteraceae stigma with yellow pollen grains attached.
From this website.

Friday, July 05, 2013

La Jencia Flash Flood!

Moving at about 5 miles per hour (as judged by floating tumbleweed), the front of the flood (visible here with lots of foamy flotsam) made a roaring sound easily audible at the ranch house.  The brunt of the thunderstorm had passed more than half an hour previous.  Based on cross sections of this reach, the flood was only about 30cm deep, and represented a flow of approximately 50 CFS.  This amount of water may be "bankfull": the bank-side sedges were underwater, the willow got their feet wet, and no major channel geomorphic changes occurred. 

Advancing front of flash flood.

 The next day.

 Flooding along old channel where side canyon empties in.  This channel was abandoned in 2009 when the creek cut through a meander bend.