Part 8

1. Part 8 PRECIPITATION OF TRAVERTINE

2. Carbon dioxide (CO2) is dissolved in groundwater percolating through limestone, forming a weak carbonic acid (H2CO3 ). This weak acid reacts with the limestone to form soluble calcium acid carbonate, CaH2(CO3). When this mixture spills over the small falls it is aerated and loses some of the dissolved carbon dioxide (CO2) to the dry atmosphere, causing the precipitation of insoluble carbonate (CaCO3) and water (H2O).

3. Precipitation of travertine at Travertine Rockin Holocene Lake Cahuilla, in the Salton Trough of the Colorado Desert of southeastern California. Lake Cahuilla was periodically filled with fresh water by the Colorado River between 700 and 1700 AD.

4. Carbonate “bathtub ring” exposed along the shoreline of Lake Mead immediately behind Hoover Dam. This ring was initially deposited during a 3-month period in Aug-Oct 1941

5. Lake Powell was formed behind the Glen Canyon Dam in southern Utah beginning in 1963. It can store up to 27 million ac-ft of water, making it the second largest man-made lake in the USA. Note the carbonate bleach line on the orange Navajo Sandstone.

6. Limnology studies in Lake Powell found that travertine deposition was stimulated by presence of algae in the warmer lake water, within 30 feet of the lake surface.

7. The limnology studies in Lake Powell also discovered fresh water diatoms in the warmer surface waters stimulated carbonate deposition and that lake salinity increased with depth.

8. Conclusions-Lake Powell Studies • Salinity increases with depth • Temperature decreases with depth, then increases near density current interface because of anaerobic digestion of organics • Dissolved Oxygen Content is replenished at depth during winter months • Silica is extracted from the lake water by diatoms • Photosynthesis in surface waters produces high pH conditions and precipitation of calcite • Calcite precipitation appears to be controlled by pH, temperature, and surface area of calcite nuclei

9. Marble Canyon is a deeply incised gorge lying between Lee’s Ferry and the Grand Canyon, carved by the Colorado River.

10. River view of Marble Canyon just downstream of House Rock Rapid (RM 17). Note what appear to be carbonate “bathtub rings” high on cliffs (arrow).

11. Cross section of Marble Canyon near River Mile 15, showing projected level of deepest lake possibly impounded by lava dams, around el. 4125 ft.This coincides with the level of the bathtub rings shown in the previous image.These reservoirs would have been short-lived, from 10,000 to possibly 30,000 years.

12. Lava Dams in Grand Canyon • During the late Pleistocene a series of lava flows built upon each other to periodically dam the Colorado River between River Miles 179 (Toroweap, above right) and 183 (Whitmore Wash, above left).

13. Extensive deposits of travertine blanket the slopes of the Grand Canyon’s Inner Gorge between River Miles

14. Vertical distribution of travertine • The travertine deposits lie between elevations of 1840 and 3440 feet, and 30 to 1655 feet above the Colorado River. • There is no apparent correlation with bedrock geology; the travertine being deposited on sandstone, shale, limestone, and dolomite. • This suggests that lake chemistry may be dominant deposition mechanism, as opposed to percolation through limestone.

15. Frequency Analysis-Travertine Deposits • Frequency analyses suggests that the travertine deposits in the Grand Canyon were likely deposited in lakes of varying elevations, in particular, those sequences between elevations 2350-2480 and 3320-3480 feet.

16. The largest of the hypothetical Pleistocene impoundments was Prospect Lake, which eventually rose to an elevation around 4125 feet, creating a series of lakes extending several hundred miles upstream, beyond present day Moab, Utah

17. River Mile 157 River Mile 57 Lava Dam • Detail view of Pleistocene Prospect Lake in the Grand Canyon. It filled the Grand Canyon to an average depth of about 1500 feet. River Mile 116