Mapping of range-piedmont-river area near the New River in northernmost Phoenix

GLG 455/598 Advanced Field Geology

February 28-March 1, 1998

This project will provide students the opportunity to map Quaternary surfaces and deposits at the urban fringe of northern Phoenix. This area is the focus of current and planned mapping efforts by the ASU graduate students and geologists with the Arizona Geological Survey, so our work will complement and contribute to those projects. Airphoto bases will be used for mapping. We plan to have some soil pits excavated in advance of the field work and the mapping will be complemented by trench logging and soil description so that students can develop a soil chronosequence to help with the relative dating of the surfaces and materials in the mapping area.

Location: West of I-17 along the north side of Carefree Highway and east of the Lake Pleasant road.

Mapping goals: Map the distribution of the different Quaternary geologic units, establish their relative ages, consider the implications for landscape development, and evaluate the recent flood history of the New River in the mapping area.

Mapping and notation strategy; Document the following:

1) Distribution of rock types. Where are the main units, what are they (describe them carefully and completely), and what did they look like when they were deposited? How were they deposited? What are the clast types? What are the sedimentary structures? It might be worthwhile to perform clast counts for each of the units.

2) Soil descriptions. Use the accompanying form to document the depth distribution of soil horizons, color, structure, texture (sand, silt, clay), gravel, carbonate, and toughness for each of the map units. Spend time in each of the holes that we have dug, as well as channel and road cuts to develop a complete soil description for each principal geomorphic surface.

3) Surface pavement development and surface roughness. What is the size distribution of clasts on the surfaces and to what degree are they varnished? How does the pavement and the meter-scale surface roughness develop over time? What processes operate over what surfaces?

4) Vegetation. Try to estimate the density and type distribution of the main vegetation types on the surfaces.

5) Landform development. What and where are the major landforms? You should map the major channels and slope breaks in the area.

6) Major landforms. Map major landforms and features in the area that may be useful to define the context.

7) Quality control and coverage. Be sure to quality control your observations. Try not to leave any gaps on the map. Make lots of careful observations reported clearly in your notebooks.

Questions to address in the write-up (Due March 31 by 5 PM to Arrowsmith)

(should be five pages plus any figures [which will include your field maps, any other sketches of key relationships that illustrate your points, including composite soil profiles for each of the main surfaces]).

1) What is the geologic history of the mapping area? What are the relative ages of the units and the landforms? What are the surface processes that have operated in the past? What are the surface processes operating there now? How, when, and where has incision and aggradation occurred in the area? How important is sediment input from local sources versus from long range transport along the New River?

2) Describe the planform and cross-sectional geometry of the landforms in the area. How stable are landforms in the area? Over the long term, how much do you think that the channels move across or up and down in the landscape?

3) How does the development of the geomorphic surfaces (including the soils) after deposition of the parent material tell you about the landscape development and climatic history? What assumptions do you have to make in that discussion?

4) What is the flood history of the New River in the mapping area? How often does it flood each of the geomorphic surfaces in the area (if ever). Look in the active channel area for materials like wood and trash that may have been deposited (stranded) during high water times in the past. Roughly estimate the depth of the largest historic flood and its extent in the channel. Which parts were dominated by high velocity, deep flow, versus those which were dominated by shallower and slower flow?

5) Based upon the above observations and inferences, where would you advise a developer to place a 100 residence development in the mapping area? What are the geologic considerations that the developer should make? What will the development do to the landscape and surface processes nearby?