Point Lobos Field Trip 3 June 06

Transcription

1 GES 56Q Monterey Bay Seminar Spring Quarter 2006 Point Lobos Field Trip 3 June 06 Meet at GeoCorner Parking Lot at 0800 Main Objectives for this trip: 1) Intertidal ecology. The low tide will occur at 11:02. We will survey the beaches along the south side of Point Lobos to find the one that is most illustrative of intertidal zonation. I want you to identify the main organisms determine their lifestyle (sessile, motile, infaunal, epifaunal, filter feeder, deposit feeder) and look for examples of both organism/organism interaction as well as environment/organism interaction. After about 1 hour, each of you should have an example to talk about. 2) Submarine canyon/fan geology. The park has superb outcrop exposures of the Carmelo Formation. They are described in the second handout for this field trip. We will look for sedimentary evidence of downslope movement of sediment as well faulting/vertical tectonics. I ll pick an outcrop somewhere and see how far you guys can go with a free-wheeling interpretation. It will help if you read this handout before we get to the park. 3) Mammal ecology. There are some elephant seals, otters, and harbor seals at Point Lobos. We may take some time to talk about otter ecology as it relates to the decline of the kelp forest and otter protection measures. However, I don t want to get back too late on Saturday.

2

3

4

5

6 Geology Page 1 of 4 5/17/2002 GEOLOGIC HISTORY 100 Million Years Ago Plate Tectonics by Jeff Thomson from "Explore...Point Lobos State Reserve" 1997 Walkabout Publications 60 Million Years Ago 20 Million Years Ago 6 Thousand Years Ago THE ROCK TYPES Sand and Pebble Beach Deposits A special thankyou to Daniel Orange, University of California, Santa Cruz, Earth Sciences Department, and Keith Simmons, Earth Sciences Department Hartnell College, Salinas, for their generous time and contributions to improving our knowledge of the special geology of Point Lobos. Marine Terrace Sediments Carmelo Formation Santa Lucia Granodiorite 100 Million Years Ago The "modern" geological history of Point Lobos begins about 100 million years ago when dinosaurs still roamed the earth. Miles below the earth's surface a molten mass of rock deep inside a prehistoric chain of active volcanoes slowly cooled into what geologists call Santa Lucia Granodiorite. Over the next 40 million years this hard, granite-like rock slowly rose to the surface and now comprises one of the four types of rocks at Point Lobos. Resting on top of the granodiorite is an ancient deposit of sand and gravel that formed about 60 million years ago and has since hardened into a sandstone called the Carmelo Formation. Lying on top of the Carmelo Formation, like frosting on a cake, is the third geologically significant type of rock found at the reserve: sedimentary rocks that were deposited on ancient marine terraces. These terraces were formed as ocean waves created wide platforms when the sea level was higher than it is today. The sediments that were eroded from the old shoreline and deposited on the marine terraces consist of clay, silt, sand and gravel up to two million years of old. The fourth type of rock at Point Lobos is found along the shoreline where you can see how wave action has worn away the Carmelo Formation and granodiorite to form deposits of gravel and white sand beaches. Plate Tectonics To best understand and appreciate the geology of Point Lobos, it's useful to have a basic understanding of plate tectonics. According to this commonly accepted geological theory, the earth's crust is divided into several large rigid blocks or plates that float like giant rafts on a core of semi-molten rock. As the earth sheds its heat, these plates move and shift into position, accounting for the occurrence of continent drift, earthquakes and volcanoes. As

7 Geology Page 2 of 4 5/17/2002 recently as 70 million years ago (a mere blip on the geologic time scale) the Santa Lucia Granodiorite here at the reserve was once located approximately 1,200 miles to the south near the tip of Baja California, Mexico. Tectonic forces then ripped away a huge block of the earth's crust containing the granodiorite, and over the next 20 million years transported it northward an extraordinary distance of almost 1,000 miles. While this large piece of rock that geologists have named the Salinian block was moving north, it was also pushed upward, temporarily exposing the granodiorite above the water and allowing rivers and streams to erode a system of valleys and canyons. Sixty Million Years Ago Around 60 million years ago, the Salinian block subsided and ocean waters filled the granodiorite canyons. On land, ancient rivers carried lava from distant volcanoes down to the shoreline, and along the way scoured it into rounded pebbles and cobblestone-sized rocks. Over time, these rocks and layers of sand were deposited on the submarine canyon walls by underwater landslides and eventually cemented into a conglomerate called the Carmelo Formation. For many people, the Carmelo Formation is the most interesting type of rock at Point Lobos. It is softer than the granodiorite and is easily seen along the south shore of the reserve, where it has been eroded by ocean waves to form a photogenic series of coves, crevices and shelves. 20 Million Years Ago The next significant period of Point Lobos geological history begins about 20 million years ago when tectonic forces once again moved this plate of the earth's crust upward and another 250 miles north to its present location. Then, with the coming of the ice ages which began around two million years ago, these rocks were exposed to the sun and eroded by rain, then submerged again as the sea level waxed and waned. When the sea stayed at one level for a long period, marine terraces and sloping cliffs along the shoreline were formed, and are still evident today. The most noticeable marine terraces at Point Lobos formed between 35 and 10 thousand years ago. The older terrace developed its shoreline at an elevation of about 125 feet near what is today Highway 1, and slopes down through the central portion of the reserve. The lowest and most recent of the old marine terraces emerged about 10,000 years go and sits at an elevation of about 40 feet above the current sea level. The short descent along the road to Whalers Cove marks this ancient shoreline. Six Thousand Years Ago Around 6,000 years ago, the ocean stabilized at its current level and began its assault on the shoreline. Waves chewed away at fractures and faults in the hard granodiorite to form the rugged north shore of the reserve. Headland Cove, and Whalers Cove are excellent examples of how pounding waves have exploited weaknesses in the granodiorite while in the process creating some of the most scenic coastline in the world. At the south end of the reserve, beautiful white sand beaches at China Cove and Gibson Beach, are composed of mineral grains from the granodiorite surrounding these coves. The story written in the rocks of Point Lobos reveals an ancient and dynamic cycle of change resulting from some of the most powerful forces of nature. It is easy to assume that what we now experience has been here forever and will always stay this way, but if we take the past as our guide, it is likely that Point Lobos will again slip into the sea. With that in

8 Geology Page 3 of 4 5/17/2002 mind, we are very fortunate to experience this grand landscape as it currently exists. The Rock Types: Descriptions by Keith Simmons, Earth Sciences Department, Hartnell College Photographs by Ed Sakai Sand and pebble beach deposits, land slide debris Recent: ten thousand years ago to today. Beach sand is composed of white quartz weathered from the Santa Lucia Granodiorite.Volcanic pebble and cobble beach deposits originated from conglomerates of the Carmelo formation. Landslide debris formed from marine terrace sediments, Carmelo formation, and granodiorite. Marine terrace sediments Pleistocene and recent: Two million years ago to today. Marine terraces are composed of clay, silt, and sand, and in places, pebbles and cobbles. The material originated from erosion of cliffs by the sea and deposition offshore on shallow wave-cut rock platforms. These gray sediments readily erode near the coastline. Carmelo Formation Paleocene: 60,000,000 years old. The Carmelo formation is composed of thousands of layers of conglomerate, sand stone, and mudstone which the turbidity currents have deposited. The mudstone was deposited as marine mud. Most pebbles in the conglomerate are of volcanic origin. Conglomerate and sandstone are shades of brown, with concentrations of reddish brown iron oxide common in the sandstone. Mudstone is gray. Fossils in sandstone and mudstone include mollusks, worm burrows, and mysterious seaweed-like crustacean impressions. The strata are folded in some locations. The Carmelo formation was deposited in a submarine canyon cut into granodiorite. Blocks of collapsed canyon walls are found within the Carmelo formation. Santa Lucia Granodiorite

9 Geology Page 4 of 4 5/17/2002 Cretaceous: 100,000,000 years old. This formation is often simply referred to as the granodiorite. It is composed of mineral grains of quartz, orthoclase feldspar, plagioclase feldspar, amphibole, and biotic mica. Phenocrysts, large one- to three-inch crystals of orthoclase, often aligned, are common. Overall, the granodiorite appears as a light-colored, speckled rock. Quartz dikes, vein-filled cracks, crisscross the rock. Joints and severe fracturing are common. Highly resistant to erosion, the granodiorite is responsible for the rugged coastline. This formation is the basement rock which underlies all of Point Lobos. HOME General Info Rules and Regs Trail Map SCUBA Natural History Cultural History School Trips PLNHA Junior Lifeguard Camping Links

10

11

12

13

14

15

16

17

18

19