Erratic Behavior on Rattlesnake Mountain, Hanford Reach National Monument, South-Central Washington

Size: px

Start display at page:

Download "Erratic Behavior on Rattlesnake Mountain, Hanford Reach National Monument, South-Central Washington"

Maria White
6 years ago
Views:

1 Erratic Behavior on Rattlesnake Mountain, Hanford Reach National Monument, South-Central Washington Bruce Bjornstad, Pacific Northwest National Laboratory Elysia Jennett, Northern Arizona University Jenna Gaston, U.S. Fish and Wildlife Service Gary Kleinknecht, Kamiakin High School Geological Society of America Annual Meeting Seattle, WA November 3, 2003

2 Area of Ice-Age Flooding Study Area Maximum extent of Lake Lewis Rattlesnake Mtn. 2

Bellevue erratic in Oregon s Willamette Valley by

3 Ice-rafted debris collected in slack- water areas along the floods path 12 Conceptualization of the Bellevue erratic in Oregon s Willamette Valley by artists Ominski and Swaren Comparable erratic on Rattlesnake Mtn. 3

4 Study Area Maximum flood level (~1200 ft.) Hanford Site 4 Hwy. 240 Flood water Rattlesnake Mtn. ~15 mi 2 Yakima R. SW Hwy. 225 NW Valley Creek Monument National Reach Hanford Cold Yakima Valley

Rattlesnake Mountain Only indigenous rock = dark Columbia River Basalt (exposed at base of arroyos) Basalt Fine-grained mantle Thin (<20

5 Environmental Setting of Study Area Pristine, undisturbed, and protected area for last 60 years, now part of the HRNM Sparsely vegetated. Totally burnt off in 2000 wildfire, and partially burnt in Gently sloping, low-relief slope of Rattlesnake Mtn. Rattlesnake Mountain Only indigenous rock = dark Columbia River Basalt (exposed at base of arroyos) Basalt Fine-grained mantle Thin (<20 ft), fine-grained mantle of Holocene loess, slopewash, and/or slackwater flood deposits above basalt Ice-rafted debris is easily recognized in this environment. 5

6 Type of debris GPS location Elevation Size Lithology Roundness Surface characteristics Data Collection Erratic size limit >1 ft 2 (Max. observed cross-sectional area) Data points = 1139 Area ~ 15 mi 2 6

7 Three Types of Ice-Rafted Debris 2. Erratic clusters 1. Isolated erratics Frequency Distribution By Type n = Bergmounds Isolated Erratics Erratic Clusters Bergmounds 7

Spatial Distribution of Ice-Rafted Debris Maximum elevation =1165 Amount of ice-rafted debris increases downslope Bergmounds restricted to 600-1000 ft elevation Buried erratics exhumed along roadways

8 Spatial Distribution of Ice-Rafted Debris Maximum elevation =1165 Amount of ice-rafted debris increases downslope Bergmounds restricted to ft elevation Buried erratics exhumed along roadways Erratics, especially in clusters, align with flooddrowned arroyos. At lower elevations erratics are concentrated along leeward, southerly facing arroyo slopes. At higher elevations they re concentrated at bottoms of arroyos. Isolated erratic Erratic cluster Bergmound Study Area Maximum flood level Floodwater 8

9 Model to Explain Limited Distribution of Bergmounds Bergmounds are not observed above 1000 ft elev., even though the floodwaters extended up to 1200 ft. The largest icebergs, required to generate bergmounds, apparently became grounded >200 ft below the maximum lake level. 9

Lithology (Isolated Erratics) 500 Number of erratics 400 300 200 100 76.

3% Spatial Distribution 0 Granitic* Quartzite Diorite Basalt Argillite

10 Lithology (Isolated Erratics) 500 Number of erratics % Frequency Distribution n = % 5.2% 3.4% 2.2% 1.6% 1.6% 0.5% 1.3% Spatial Distribution 0 Granitic* Quartzite Diorite Basalt Argillite *Granitic = granite or granodiorite Schist Gneiss Maximum Gabbro Other flood level Banded Argillite 10

Size of Erratics* 400 300 31.6% 28.2% Frequency Distribution n = 1139 21.3% 200 100 13.4% Spatial Distribution 0 1.0% 0.

11 Size of Erratics* % 28.2% Frequency Distribution n = % % Spatial Distribution 0 1.0% 0.4% >64 Size Class (Max. cross-sectional area in ft 2 ) Maximum flood *isolated erratics and maximum size erratic in groupings level 11

Surface Weathering (Isolated Granitic Erratics) Number of

18% 47% Moderate 34% Strong Weather-resistant, resistant,

weathered, spalled, granodiorite surface Unweathered

12 Surface Weathering (Isolated Granitic Erratics) Number of Erratics Unweathered n = 383 1% Slight 18% 47% Moderate 34% Strong Weather-resistant, resistant, raised, plagioclase phenocryst Close-up of strongly weathered, spalled, granodiorite surface Unweathered granodiorite Strongly weathered granodiorite with desert varnish 12

Number of erratics 200 150 100 50 Roundness

13 Number of erratics Roundness (Isolated Erratics) 33% 33% n = % 14% 0 Angular Subangular Subrounded Rounded Angular, unweathered diorite Rounded, weathered granodiorite 13

14 Could Weathering and Roundness be an Indication of Time Since Flooding? There is a general trend for fresh, unweathered erratics to be more angular - and weathered ones to be more rounded. This may suggest that roundness is more a function of in-situ weathering than abrasion. * Some weathered and rounded erratics might be expected to predate rafting, but probably not 50% as observed on Rattlesnake Mtn. Therefore, a significant number of rounded and weathered erratics might be from pre- Wisconsin flood events. * Number of isolated, granitic erratics (n=378) 14

15 Summary Over 95% of erratics are exotic, and over 75% are granitic; rock types are consistent with sources >100 mi away, N and NE of the Columbia Plateau. An increase in the amount of ice-rafted debris downslope is consistent with a greater number of less-than-maximum floods. Most erratics are small, 60% are less than 2ft X 2ft in plan view. An increased number of erratics lie along roadways, exhumed during road construction. This suggests that many more erratics than appear on the surface, lie buried within the fine-grained soil mantle. Distribution patterns are different for isolated erratics, erratic clusters, and bergmounds: Alignment of erratics along arroyos could be due to: 1) eddy currents during flooding, created by variable flow velocities across an uneven surface, or 2) exhumation via stream erosion along the arroyo. Bergmounds don t extend above 1000 ft elev., probably because larger icebergs naturally became grounded on the gently sloping lake bottom further from the ancient lakeshore. Most erratics from the recent floods might be expected to have only angular, slightly weathered surfaces. However, over 50% of erratics are rounded to subrounded. This, combined with the observation that >80% of erratics are moderately to strongly weathered, suggests that a substantial number may be from older, pre-wisconsin floods. 15

Day 3 Weathering and Erosion.notebook. October 02, Section 7.2. Erosion and Deposition. Objectives

Day 3 Weathering and Erosion.notebook. October 02, Section 7.2. Erosion and Deposition. Objectives Objectives Describe the relationship of gravity to all agents of erosion. Contrast the features left from different types of erosion. Analyze the impact of living and nonliving things on the processes