GLACIAL EROSION AND STRATIGRAPHY

Transcription

1 GLACIAL EROSION AND STRATIGRAPHY Objectives: In this lab we will use concepts of glacial erosion and deposition to illuminate the glacial history of the Puget Sound region. After this lab you should: 1) understand the use of erosional and depositional features in determining ice flow directions; 2) understand the concepts and problems of applied glacial stratigraphy; and 3) have a general knowledge of the glacial history of the Puget Sound region. Key Terms and Concepts: Glacial Striae Drumlinoid Landforms Sediment Provenance Glacial Drift Package Composite Stratigraphic Section Eustasy Isostasy Ice Dammed Lake Glaciomarine Drift Fjord Part 1. Indicators of Glacier Flow Direction: A variety of evidence can be used to determine flow direction(s) of glaciers across an area, including glacial striae, drumlinoid landforms, and sediment provenance. A) Glacial Striae As a glacier or ice sheet passes over bare bedrock, rocks entrained in the underside of the glacier (glacial "tools") scratch the bedrock, producing striations, or striae. The orientation (longitudinal axis) of the striae indicates the direction of glacier flow across that area. In many locations, striae cross-cut one another, suggesting multiple flow directions. At such locations, it is often possible to discern relative ages of the striae by determining which striation direction cuts across the other. 1) In what way is the flow direction derived from a set of striae ambiguous? 2) Examine the glacial "tools" (the scraper rocks from the bottom of the glacier). If you were looking at a pile of rocks in a glaciated area, how would you identify a glacial tool? 1

2 3) Examine the ice-flow indicator directions shown on the Puget Lowland DEM. Why do the directions vary? What happened to the ice sheet in the marked area lying southeast of Seattle? B) Drumlinoid Landforms Drumlins and drumlinoid features are formed by as-yet poorly understood processes beneath ice sheets. These elongate hills indicate the direction of glacier flow across the area. Recall from Lab 1 that the orientation of the drumlin can be used to reconstruct ice-flow direction of an ice sheet. 1) Examine the drumlinoid landforms on the Seattle topographic map. What was the direction of glacier flow in this area? 2) Examine the drumlinoid landforms on the Maple Valley topographic map. What direction was the ice flowing here? 3) Are the ice flow directions indicated on the Seattle and Maple Valley maps similar? Explain any similarities or differences. Look at the small laminated DEM image of the Puget Lowland and consider the geometry of the ice sheet. What determines the direction of ice flow of a large ice sheet that inundates topographic features of an area? C) Sediment Provenance The provenance, or source region, of rocks found in glacial deposits can be a valuable tool in discerning regional ice flow directions. For provenance to be useful, possible source areas for a glacial deposit must have distinct lithologies. In answering the following questions, you may want to consult a geologic map of the U.S. 2

3 1) On the side table are clasts from three different tills (Till A, Till B, and Till C). First, identify the major rock types included in each till and then, using a geologic map and your knowledge of northwest geology, determine a source region for the three till samples: Fraser Valley (Canada), Mt. Rainier, or the Olympic Mountains. The rocks of the Fraser Valley are similar to those found in the North Cascades. Next, determine which site (Sites 1, 2, or 3 on the large maps) each till came from and, finally, what depositional agent was responsible for depositing each till (Cordilleran Ice Sheet, Olympic Alpine Glaciers, Mt. Rainier Alpine Glaciers). Till Rock Types Source Region Site Depositional Agent A B C 2) The Juan de Fuca lobe of the Cordilleran Ice Sheet carved out the Strait of Juan de Fuca. Near Forks, on the northwestern Olympic Peninsula, the deposits of the Juan de Fuca lobe merge with deposits of alpine glaciers from the Olympic Mountains. How could you distinguish the ice sheet and alpine glacier deposits (i.e. what rock types would you look for)? Part II. Eustatic and Isostatic Effects on Glacial Erosion and Deposition During glaciations, a substantial proportion of the Earth's water is locked up in glacial ice covering much of northern North America and northern Eurasia. Since the ultimate source of the ice is ocean water, worldwide sea level falls during glaciations and rises during interglaciations. These changes in sea level are called eustatic fluctuations and are global in extent. Such fluctuations complicate the glacial history of a marine area such as the Puget Lowland. As if that weren't enough, isostasy must also be taken into account. As an ice sheet burdens the Earth's crust, the crust sinks into the mantle to compensate for the added weight, and rises when the ice sheet disappears. This process 3

4 is analogous to a ship riding lower and higher in the water when it is fully loaded and empty, respectively. Figure 1: Eustatic (global) sea level rise due to ice sheet ablation following deglaciation (18-6 ka years ago). A) Eustasy 1) In lab 1, you learned that fjords are steep-walled valleys, carved by valley glaciers and filled partially with sea water. With the concept of eustasy in mind, speculate on the abundance of fjords in many formerly glaciated coastal areas (i.e. what is the sequence of events leading to the formation of a fjord?). 2) Examine the topographic map of the Olympic Peninsula. What would the Olympic Peninsula look like if sea level rose 300 meters (~1000 feet)? 4

5 B) Isostasy Figure 2: Schematic representation of mantle displacement and recovery during and following ice loading during a glacial cycle. 1) As the Cordilleran Ice Sheet withdrew from Puget Sound, marine water eventually entered the area, and the ice sheet discharged sediment that was deposited as glaciomarine drift, during the Everson Interstade. At the Double Bluff locality on Whidbey Island, these glaciomarine deposits lie 5 meters above modern sea level. Similar deposits were deposited north of Vancouver, British Columbia, as marine waters entered the Strait of Georgia. There, the deposits lie about 150 meters above modern sea level, even though they are of similar age and eustatic sea level would have been the same at both locations. What is the likely cause of this disparity in elevation of the deposits between Whidbey Island and the Strait of Georgia area? 5

6 Part III. Glacial Stratigraphy of the Puget Lowland The Puget Lowland has experienced numerous advances and retreats of the Cordilleran Ice Sheet. Different portions of the glacial record are preserved in different areas of the Lowland. Stratigraphic sections of Quaternary deposits are shown at three locations: Whidbey Island, Discovery Park (Seattle), and the southern Puget Lowland. A) Whidbey Island exposes deposits of at least three ice sheet advances, named the Double Bluff, Possession, and Vashon glaciations. Separating the glacial tills are peat (complete with compressed logs) and fluvial (river-deposited) sands, deposited during non-glacial periods (the Whidbey Interglaciation and the Olympia Interstade (Kitsap Formation)). 1) Two Whidbey Island stratigraphic sections are provided with your lab materials. Combine these two sections into one composite section by correlating overlapping sediment units. B) Discovery Park (Seattle) The bluff exposures at Discovery Park, Seattle comprise deposits associated with the last non-glacial to glacial cycle. The Kitsap Formation consists of fluvial sands deposited by low-gradient (not steep), northward-flowing rivers. The Lawton Clays are lake deposits, overlain by the Esperance Sand (advance outwash deposits of the Puget Lobe) and the Vashon Till. In topographic lows or troughs, the Vashon Till is overlain by retreat outwash and lacustrine sediments. This typical cycle of sediments (diagrammed in Figure 3), called a glacial drift package, is found at many locations and from several different glacial episodes in the Puget Lowland. 6

7 Figure 3: Idealized Glacial Drift Package (from one glaciation) youngest Interglacial Sediments oldest Postglacial Lake Deposits Recessional Outwash Deposits Glacial Till Advance Outwash Deposits Proglacial Lake Deposits Interglacial Sediments 1) What caused the progression from north-flowing streams (Kitsap Formation) to organic-poor lakes (Lawton Clay) in the Puget Lowland? 2) How might a glacial drift package in eastern Washington, where the rivers flow to the south, differ from the Puget Lowland package? 3a) Many river valleys of the Western Cascades contain lacustrine and deltaic deposits (at Monroe on the Steven's Pass highway, for example). What might have caused these lakes to form? How do deltas form? In what other areas in the Puget Lowland region would you expect to find such lake and delta deposits? b) How could you tell if a delta formed in a proglacial or a postglacial lake environment? 7

8 4a) The Kitsap Formation extends from sea level to about 5 meters elevation at Discovery Park. It also underlies the Kitsap Peninsula at about the same elevation. What does this modern distribution of the Kitsap Formation imply about the former extent of the formation? (Hint: Think of the environment the Kitsap was deposited in.) b) What happened to the Kitsap Formation between Discovery Park and the Kitsap Peninsula, since the time it was deposited? c) The rivers flowing into the Puget Lowland are currently forming deltas, depositing large volumes of sediment into the Sound. If the next glaciation were to begin 50,000 years in the future, what might the Puget Lowland look like before the onset of ice sheet glaciation? C) Southern Puget Lowland Exposed in the southern Puget Lowland are several glacial and non-glacial units that have a reversed magnetic polarity, overlain by the Vashon Till. Note the Salmon Springs portion of the section. 1) What is the break in the stratigraphic record between the Salmon Springs Drift and the Vashon Till called? What does it represent? 2) Name two possible reasons for the absence of Double Bluff or Possession Drift in the southern Puget Lowland. D) Composite Puget Lowland Stratigraphic Section In an area as large as the Puget Lowland, it is unlikely you will find a single exposure or area that reveals the entire glacial sequence. It is possible, however, to construct a composite stratigraphic section for the Puget Lowland, with some time gaps represented, by correlating between stratigraphic sections at different locations. 8

9 1) Using the Whidbey Island, Discovery Park, and southern Puget Lowland stratigraphic sections, compile a composite section for known glacial drifts of the Puget Lowland. Note all known absolute dates, and mark unconformities with a wavy line. 2) Based on the above section, how many times did glaciers advance into Puget Sound? 3a) From the stratigraphic column determine when the ice sheet reached Whidbey Island, Seattle and Olympia during the last glaciation (represented by the Vashon Till). Then determine when the ice sheet receded past each of these locations? Whidbey Island Seattle Advanced to this location by: Receded back past here by: 9

10 Olympia (South Puget Lowland) b) At what rate did the ice sheet advance? The distance between Whidbey Island and Olympia is approximately 65 miles. Give you answer in feet per year. (5280 feet = 1 mile) Show your calculations. c) At what rate did it recede? d) What do these rates tell you about the relative rates of ice sheet advance and retreat? 10