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The Geologic Evolution of Point Lobos The rocks at Point Lobos originated far away, under very different conditions from those that prevail here today. Their origin and translocation involves the interaction of great plates of the earth's crust. Plate tectonics, the term commonly used to describe the effects of this interaction, derives from heat variations within the earth's mantle. These variations are thought to drive huge convection cells within the mantle (Fig. 1). Upward-rising mantle creates large cracks in the earth's lithosphere, oceanic ridges that are the site of volcanism and new crust formation (Figs. 1 and 2). The new crust moves away from the oceanic ridge and forms a plate which extends across the ocean floor until it encounters another plate (Fig. 3). The oceanic ridges are major features of the earth’s surface and can span entire oceans (Fig. 4). The ridges are broken by large fractures in the crust called transform faults (Fig. 4), across which the ridges step sideways and newly created crust moves laterally (Fig. 5). LITHOSPHERE MANTLE EARTH'S CORE Clifton, 2010 Figure 1. Cross section of the earth showing the nickel-iron core, the extremely dense plastic mantle and the rigid outer shell, the lithosphere. Convection cells in the mantle cause the lithosphere to crack and spread apart. Figure 2. Cross-section of the outer part of the earth where mantle material wells up in a crack on the earth’s surface. The cracks occur predominantly in the ocean and form extensive oceanic ridges. Newly formed crust spreads away from the crack, giving rise to the term “sea-floor spreading”. 1 OCEANIC PLATE CONTINENTAL PLATE OCEANIC RIDGE CONTINENTAL CRUST (CRATON) OCEANIC CRUST AND LITHOSPHERE Eurasian Plate Figure 3. Idealized oceanic plate extending from an oceanic ridge to an adjacent continental plate. Eurasian Plate Plates global Indian Plate North American Plate Arabian Plate Pacific Plate Cocos Plate Phillipine Plate Indo-Australia Plate South American Nazca Plate Plate OCEANIC RIDGE African Plate Antarctic Plate Figure 4. Major lithospheric plates of the world. Inactive (passive) continental margins shown in blue; active margins in red. Active margins consist of relatively young rocks and are subject to earthquakes and volcanoes. Dark red lines denote oceanic ridges, which are broken by fracture zones and transform faults (dark heavy lines). Modified from Tilling, Heller and Wright (1987) and Hamilton (1976). 2 Transform fault FRACTURE FRACTURE ZONE The geologic history at a continent’s margin depends greatly its position relative to a plate boundary (Fig. 4). Many continental margins, such as those on the east side of North and South America, most of Africa and all of Australia lie well within a plate. Such margins are geologically “quiet” sites of sediment accumulation. Other continental margins coincide with the boundary between crustal plates. Rock here is continually being created and destroyed. Earthquakes and volcanoes are common features. The geologic history on an active margin is further controlled by the nature of the interaction. Where the boundary between the plates parallels the transform faults, the plates slide sideways relative to each other. Such a margin is called a “transform margin” (Fig. 7). TRANSFORM ZONE TRANSFORM FAULT FAULT OCEANIC RIDGE OCEANIC OCEANICRIDGE RIDGE ! FRACTURE ZONE Oceanic Crust Mantle FRACTURE ZONE OCEANIC RIDGE Figure 5. “Transform faults” and “fracture zones are giant cracks in the earth’s crust. In the ocean basins, oceanic ridges are commonly displaced by transform faults. The ridges are not necessarily moving apart, but the crust on opposite sides of the fracture may be moving in opposite directions or, in fracture zones, in the same direction, but at different rates and elevations. Figure 6. Oceanic crust, composed of heavier elements, differs significantly in thickness from continental crust, which consists of less dense minerals. Part of the reason for this difference in thickness is that oceanic crust is continually being formed and is destroyed before it can accumulate to great thickness. The oldest oceanic crust known is about 270 million years old, much younger than continental crust, which can be billions of years old. 100 km (60 mi) 3 Where the plates collide head-on a different process occurs. If the other plate consists of continental crust, the thinner, more dense oceanic plate tends to slide beneath the continental plate in a process called subduction. The force of the collision generates heat and pressure; water carried into the system reduces the melting point of the rocks; five to ten miles below the surface of the earth, they can become a fluid magma (Fig. 8). Figure 7 shows the distribution transform and subducting margins on the western sides of North and South American continents. Central and southern California lie on a transform margin on which the Pacific Plate is sliding to the northwest relative to the North American Plate. Much of Northern California and the Pacific Northwest lies on a subducting margin. It is no accident that volcanoes in the “Ring of Fire” around the Pacific Ocean lie along the subducting margins: the Andes, Central America, the Pacific Northwest and the Aleutian Chain (Fig. 7). While damaging earthquakes can occur anywhere on an active margin, areas of subduction are most likely to suffer giant, tsunami-generating earthquakes. SUBDUCTION TRANSFORM (SLIDING) MARGIN SUBDUCTING (COLLISIONAL) MARGIN Figure 7. Plate boundaries on the eastern side of the Pacific Ocean. In some areas the plates are sliding past one another (“transform margin”). In others the plates are colliding head-on (“subducting margin”). The nature of the boundary controls the geological processes and history. CONTINENTAL CRUST Figure 8. Subduction results where oceanic crustal plates collide head-on with continental crustal plates. The denser oceanic crust is driven beneath the lighter continental crust. Water carried by this process into the depths of the earth lowers the melting point of the rocks. The resulting molten rock (magma) can vent to the surface in the form of volcanoes. . OCEANIC CRUST 4 Most geologist today agree that Salinia is a southern extension of the Sierra Nevada that was rifted north by movement on the San Andreas Fault. The processes whereby this happened are shown in the following sections. The geology of the Central California Coastal area long mystified geologists because it consists of a large elongate block of continental crust (the Salinia Terrane) (Fig. 9) encased in oceanic crust. In the 1980’s a group of geologists thought that remnant magnetism in this rock indicated it originated from near the equator. Their subsequent analyses, however, did not support this hypothesis, and it was abandoned. SN SIERRA NEVADA BATHOLITH SA SALINIA TERRAIN TERRANE SFB OCEANIC CRUST M - Monterey M SN SA SB - Santa Barbara SFB - San Francisco Bay SAF - San Andreas Fault - Monterey Peninsula Faults PACIFIC OCEAN SAF SB Figure 9. A large block of continental crust, called the “Salinia Terrane” (SA on this figure) long puzzled geologists because it lay sandwiched between masses of oceanic crust. 5 The western margin of North America has long been the battleground between the continental plate and a series of large oceanic plates. For many tens of millions of years, a large oceanic plate, the Farallon plate, collided with the North American continent, generating a linear chain of large magma chambers (masses of molten rock). These masses cooled and were later uplifted to become the rocks that form the presentday Sierra Nevada Range. Eighty million years ago (80 Ma), the North American continent was split in two by a large northsouth seaway (Fig. 10). In the area of present-day Southern California and northern Mexico, magma chambers created by subduction of the Farallon plate were cooling, forming an igneous rock. Somewhere, probably in northwestern Mexico, granodiorite was crystallizing 5 miles or more below the surface of the earth (Fig. 11). This rock would later be exposed as the granodiorite at Point Lobos and the Monterey Peninsula. Figure 10. Inferred paleogeography of North America and the North Pacific in the Late Cretaceous Period, around 85 million years ago. A large seaway splits the continent. On the western continental margin, the large Farallon oceanic plate collides with the North American plate, creating subduction all along the margin (redrawn from Orr and Orr, 2002). Dashed line: current outline of North America. Figure 11. Eighty million years ago, a magma of molten rock material created by subduction was cooling in what probably today is northwestern Mexico. The newly crystallized solid rock will be called “granodiorite” because of its mineral composition. It is this rock that underlies Point Lobos and the entire Monterey Peninsula today. Dashed white lines: current western state boundaries. Red “X” indicated the possible location of what would become Point Lobos. The compilation of this and other paleogeographic maps show here is discussed briefly at the end of this paper. 6 How do we know when the granodiorite crystallized? The systematic decay of trace amounts of radioactive potassium in the feldspar provides a clock for dating the time of crystallization. Analyses indicate that this occurred during the Late Cretaceous period, nearly 80 million years ago (Kistler, R. W., and Champion, D. E., 2001, U. S. Geological Survey OpenFile Report 01-453). Volcanoes may have erupted above the chamber, but their existence has since been lost to erosion. upward along a large thrust fault. In any event, lying deep beneath the ocean’s surface, the granodiorite was covered by mud and rocky debris that slid down the ancient undersea slope. The resulting deposits are reflected by the shale and limestone breccia beds exposed at Gibson Beach. Rare, poorly preserved fossils in this rock indicate a late Paleocene age (Bowen, 1965). The next geological event that is recorded at Point Lobos probably occurred in the late Paleogene Epoch, some 20-25 million years after the granodiorite crystallized (Fig. 12). By this time the granodiorite had shifted from its position miles below the earth’s surface and now lay exposed on the continental margin (Fig. 13). How this happened is still debated among geologists. Some think it was pushed westward and granodiorite 25-30 my Figure 13. In the late Paleocene Epoch, 55-60 million years ago, mud accumulated on top of granodiorite exposed on the continental (or basin) slope. Consolidated long ago into rock, this material is exposed today in the cliff behind Gibson Beach. Red “X” indicated the possible location of what would become Point Lobos. Figure 12. After the granodiorite crystallized nearly 80 million years ago, the next geological event recorded at Point Lobos appears to be in the late Paleocene Epoch, 55-60 million years ago. Dinosaurs no longer existed, having died out 65-66 million years ago at the end of the Cretaceous Period. 7 the early Eocene granodiorite 25-30 my Figure 14. Most of the Carmelo Formation at Point Lobos consists of submarine canyon fill. The few microfossils found in this sediment indicates that it accumulated in the early Eocene, some 50-55 million years ago. Although the sedimentary rocks at Gibson Beach are considered part of the Carmelo Formation (a term given to deepwater sandstone, shale and conglomerate deposited in this area during the Eocene and Paleocene epochs), they differ markedly from the rest of the Carmelo Formation at Point Lobos, which is mostly conglomerate and sandstone. Fossils are very scarce in this latter rock, but microfossils collected from mudstone at Hidden Beach indicate deep (600 to 2000 feet) marine deposition during the early Eocene (Clark, 1997) (Fig. 14). Sedimentary features in this rock similarly support a deepwater origin. Since the sandstone and conglomerate fill a valley cut into the granodiorite, the deposit is considered to represent the filling of a submarine canyon cut into an Eocene continental slope that existed in what is now northwestern Mexico (Fig. 15). Continued sedimentation buried the sand and gravel that accumulated within the submarine canyon. Under the weight of this overburden, the sand, gravel and mud gradually converted to rock: sandstone, conglomerate and mudstone, the material we see today at Point Lobos. Figure 15. During the early Eocene, gravel, sand and mud accumulated in the submarine canyon that existed on the ancient continental margin in what today is Southern California. Red “X” indicated the possible location of what would become Point Lobos. 8 So far in our reconstruction, the interaction of the North American continental plate and the oceanic plate on its western margin has consistently been one of collision; subduction has reigned all along the west coast of North America. That, however, was about to change in the late Paleogene period. Another oceanic plate, the Pacific Plate, was approaching the coast from the south and the Farallon Plate was being consumed by subduction. As the oceanic ridge that separates the Farallon and Pacific plates (known today as the “East Pacific Rise”) approached the continent, Fig. 16). At some time during the Oligocene Epoch, between 23 and 34 million years ago, the Pacific Plate encountered the North American Plate. The Farallon Plate split into two segments: the Juan de Fuca Plate to the north and the Cocos Plate to the south. As the Pacific Plate moved north, it dragged against the continental plate. As a consequence the East Pacific Rise (our term today for the oceanic ridge on the eastern side of the Pacific Plate) and associated transform faults rotated in a clockwise direction along the continental margin. The right- lateral displacement (the right side moves toward you) that characterizes the boundary between the Pacific and North American plates today was initiated. Figure 16. Around 35 million years ago, in the early Oligocene Epoch, the Farallon Plate was being consumed by the process of subduction and the northeastern edge of the Pacific Plate approached the North American continent. Figure 17. By the late Oligocene, the Pacific Plate had encountered the North American continent and the Farallon Plate split the smaller plates Juan de Fuca and Cocos Plates. Transform faults become parallel to the boundary between the two plates and a transform margin begins to develop. 9 At some point during this initial collision between the Pacific and North American Plates, a volcano spewed lava (“basaltic andesite”) over what would become the Monterey Peninsula. Radiometric dating places the time of eruption at 27 million years ago, during the Oligocene Epoch (Fig. 18). At that time, the Eocene canyon fill and the Cretaceous granodiorite had been uplifted and were exposed at the earth’s surface. The basaltic andesite is not exposed at Point Lobos, but it does crop out at short distance away at either end of Carmel Beach (Fig. 19). It probably covered the Carmelo Formation and the granodiorite at what would later become Point Lobos, but has since been eroded away. Oligocene Volcanism (27 my) 25-30 Carmelo Fm granodiorite Figure 18. Twenty-seven million years ago, this area was the site of volcanic activity Figure 19. Basaltic andesite outcrop at the north end of Carmel Beach. Figure 20. Twenty-seven million years ago, volcanic rock lie at the earth’s surface. It is unclear when the volcanism occurred relative to the collision between the North American and Pacific plates. The cartoon here shows it happening well after the initial encounter, but this is pure speculation. Red “X” indicated the possible location of what would become Point Lobos. Photograph by John Lyons. 10 As the boundary between Pacific and the Farallon Plates shifted northward, the transform margin separating the Pacific Plate from the North American Plate expanded. The timing of the next event that is recorded in the rocks of this area is very poorly constrained. Across Carmel Bay from Point Lobos, a striking granitic boulder conglomerate lines the northeastern shore of Stillwater Cove. Fragments of basaltic andesite in the conglomerate document its age to be less than 27 million years, but how much less is uncertain. It is older than the middle MIocene sandstone that underlies Carmel Beach. In this discussion, I refer to it only as “early Miocene?” (Fig. 21). Early Miocene? alluvial fan alluvial fan volcano Carmelo Fm granodiorite Figure 21. At some time following the Oligocene volcanic eruption, but before the middle Miocene shallow water sands accumulated, a coarse alluvial fan composed of granitic boulders existed in this area. The poorly-sorted, coarse character of the conglomerate resembles that of gravel in modern alluvial fans, and the size of the clasts (some more than a meter across) indicates a nearbyl source. A crude imbrication (shingling) of the clasts suggests that the highlands lay to the north. Figure 23. At some time following the Oligocene volcanic eruption, but before the middle Miocene shallow water sands accumulated, a coarse alluvial fan composed of granitic boulders existed in this area. Red “X” indicated the possible location of what would become Point Lobos. Figure 22. At some time following the Oligocene volcanic eruption, but before the middle Miocene shallow water sands accumulated, a coarse alluvial fan composed of granitic boulders existed in this area. 11 As the transform margin continued to expand into the middle Miocene (11.6-16 Ma), segments of the North American Plate became detached and began to move north with the Pacific Plate. The rocks that were to compose Point Lobos were probably incorporated into this movement. Middle Miocene shoreline coastal bay During the earlier part of the mIddle Miocene (Fig. 24), a shoreline passed across this area (Fig. 25). Sandstone and mudstone that crop out on Carmel Beach (Fig 26) contain physical features and trace fossils characteristic of a tidedominated coastal bay. Such bays are typical of subsiding coasts where the sea is encroaching over the land. fan volcano Carmelo Fm granodiorite Figure 24. The next event that is recorded in the vicinity of Point Lobos is the passage of a shoreline across this area in the earlier part of the middle Miocene Epoch Figure 26. Exposure of sandstone on Carmel Beach after a storm. The freshly exposed rocks on the beach contain trace fossils typical of a shallow marine environment, evidence of reversals of current flow typical of tides, and mudstone that contains tiny fossil rhizomes typical of a salt marsh. These features in aggregate indicate deposition in a tide-dominated coastal bay. Figure 25. During the middle Miocene Epoch, the Point Lobos area lay on or near a shoreline formed as the sea encroached over the land. The rocks that underlie Carmel Beach contain evidence of tidal currents and shallow marine organisms typical of a coastal embayment. Red “X” indicated the possible location of what would become Point Lobos. Photo by J. M. Sullivan 12 This “marine transgression” may have been the beginning of the development of a fairly deep marine basin in this area. Around 10-13 million years ago, the margin of Southern California and northwestern-most Mexico consisted of a series of marine basins (Figs. 27 and 28). The middle Miocene was a time of Middle Miocene marine basin marine basin extraordinary biologic productivity in the waters off this part of North America. Basins along the margin (Fig. 28) were floored by accumulations of the tests of microscopic marine organisms, particularly diatoms, whose siliceous tests accumulated in great thickness on the sea floor. The rocks exposed today at Point Lobos lay beneath on of these basins. Under the weight of accumulating sediment, the siliceous mud transformed into a siliceous mudstone that we today call the “Monterey Formation. shore fan volcano Carmelo Fm granodiorite Figure 27. Some 10-13 million years ago, the area of Point Lobos lay beneath the floor of a biologically highly productive marine basin. This rock once blanketed the whole area here. It no longer crops out at Point Lobos, but isolated small boulders on the Reserves shoreline and pebbles in Pleistocene gravel document its former presence here. Light-colored outcrops in the hills and road cuts around Monterey and Carmel document its continued presence in the area. If appropriately cemented, it forms a building stone (“Carmel stone”) that is much utilized on the Monterey Peninsula (Fig. 29). Figure 28. Later in the middle Miocene, the margin of southern California and northern Mexico consisted of a series of marine basin in which great thicknesses of diatomaceous mud accumulated. Red “X” indicated the possible location of what would become Point Lobos. Note that it is shown here in what today is southern California Figure 29. “Carmel stone” (in the wall of a structure in Monterey) is a building stone composed of especially well-cemented siliceous mudstone of the Monterey Formation. 13 A discussion of the evolution of Point Lobos would not be complete without discussion of the San Andreas Fault. This huge fracture forms the boundary between the Pacific and the North American Plates in most of California today (Fig. 30). At its northern end, it merges with the Mendocino Fracture Zone, which separates the Pacific Plate from the Juan de Fuca Plate. In Southern California the San Andreas extends to an area east of Los Angeles where it splits into several faults that trend toward the Gulf of California. Central California contains several large faults that are part of the San Andreas system; the one closest to Point Lobos is the San Gregorio Fault which lies a few miles offshore from the Reserve. San Andreas Fault Subsidary faults Mendocino Fracture Zone Location of 23 million-year-old volcanic rocks displaced by the San Andreas Fault The distance of offset of rocks along the San Andreas Fault in central California is generally accepted as 315 km (195 mi), based on the correlation of 23 million-year-old rocks at the Pinnacles National Park with similar rock of that age on the opposite side of the fault near Lancaster, Ca. (Fig. 30). Assuming a rate of northward movement of the Pacific Plate of 2 inches/ year (a typical estimate), an offset of 195 miles would be accomplished in about 6 million years. This implies that prior to ± 6 million years, stresses inherent in the oblique collision of the North American and Pacific plates generated a northerly transport of fragments of the continental plate via a network of shorter (and now largely unidentifiable) faults. The activation of the San Andreas fault largely consolidated that transport into a single great block lying between it and the ocean. Figure 30. Location of the San Andreas Fault (red) and subsidiary faults that currently define the boundary between the Pacific and North American Plates. (Source: USGS, Shultz and Wallace, 1997) 14 Late Miocene shallow marine shallow fan marine basin shore volcano Carmelo Fm granodiorite Figure 31. Six million years ago, in the late Miocene, the rocks that would form Point Lobos lay beneath of a shallow sandy sea floor. Figure 32. Six million years ago, the rocks that would form Point Lobos lay beneath of a shallow sandy sea floor 180-200 miles southeast of their present location. In the late Miocene, 6 or so million years ago, some of the marine basins had ceased to subside and were filling with sediment (Fig. 31). Our Point Lobos rocks were still far to the south-southeast of their present location. The San Andreas Fault is a newly-formed boundary between the Pacific and North American Plates, and the Gulf of California is just beginning to open (Fig. 32). During the Late Miocene, the San Gregorio Fault, a subsidiary fault to the San Andreas, began to move the rocks just seaward of Point Lobos to the northwest. Conglomerate at Point Reyes has been correlated with the Carmelo Formation Burnahm, 2005) Figure 33. Two million years ago, the rocks that would form Point Lobos remained buried and lay some 60-80 miles south of the Reserve. Strata on the west side of the San Gregorio Fault (SGF) was being transposed to the north-northwest By the early Pleistocene, 2 million years ago, the rocks that will be exposed at Point Lobos were deeply buried about 60-80 miles south of their current exposure (Fig. 33). 15 The Pacific Plate continues today to move north-northwest relative to the North American Plate. The stresses imposed by the grinding of these two great plates against one another create “wrinkles” (think of putting your hands palm down on a table cloth and simultaneously moving one toward you and the other away from you). The resulting “wrinkles” are manifested in the mountain ranges and basins that lie along the coast (Fig. 34), most of which have developed in the last few million years, and continue to actively grow today. Coast Range PACIFIC OCEAN Ben Lomond Mt. Madonna UPLIFT Santa Cruz Watsonville DOWNWARP LAST FEW HUNDRED THOUSAND YEARS Sedimentary deposits indicate that the area along the coast north of Santa Cruz and south of Monterey has been rising during the past several hundred thousand years, in contrast to the Monterey Bay plain, which mostly has been subsiding. I estimate that the rocks at Point Lobos are currently being elevated at a rate of 6-7 inches per 1000 years, based on the elevation of the lowest marine terrace at the Reserve (see following discussion). Moss Landing Salinas Fremont Peak Gabilan Range UPLIFT Mt. Toro If this rate of uplift remained fairly constant through time, a million years ago the rocks here would have been some 500-600 feet lower, and the coastline would likely have been formed by the Monterey Formation. As uplift progressed the Monterey was removed by erosion and the Carmelo Formation and the granodiorite emerged. It is probable that the physical shoreline feature that we know as Point Lobos came into existence several hundred thousand years ago. 0 25 MILES 0 40 KILOMETERS Santa Lucia Range Figure 34. Forces associated with the grinding of the Pacific and North American Plates have created huge “wrinkles” that result in the coastal mountain ranges and lowlands. The upward and downward movements that respectively produced the ranges and lowlands are still active today. The Pleistocene (“ice ages”) MIOCENE EPOCH The Pleistocene Epoch (Fig. 35) was a time of repeated glaciation. Massive ice sheets, 1-2 miles thick, several times covered then retreated from northern North America and Europe. The water that composed these great glaciers came from sustained snowfalls that were ultimately fed by the ocean. The transfer of this water out of the ocean caused a global fall of sea level. The transferral of water back to the oceans when the glaciers melted coincided with a rise in sea level. Marine basin bay 0 OLIGOCENE volcanism EOCENE EPOCH PALEOCENE Submarine canyon 50 MILLIONS OF YEARS AGO LATE CRETACEOUS granodiorite 100 Figure 35. The Pleistocene Epoch extends from the present day back to about 2.6 million years ago. It was the time of glaciation (ice ages) on the earth. 16 As a result, Pleistocene sea levels fluctuated between lowstands during glacial episodes and highstands during interglacial times (Fig. 36). At the last lowstand (about 17,000 years ago), sea level lay about 130 m (more than 400 feet) below its present level. These fluctuations left their mark on the coast of Central California, especially in areas of uplift (Fig. 34). LATE PLEISTOCENE SEA LEVEL SEA LEVEL (METERS) 0 50 When sea level remains constant for a long period of time, the waves cut a very gently-sloping platform that extends from the shoreline to a depth of about 30 feet (Fig. 37). The long periods of sea-level highstand shown in Figure 36 are sufficient to produce wave-cut platforms into the rocks along this coast. 100 500 400 300 200 100 today THOUSANDS OF YEARS AGO While the late Pleistocene sea level was rising and falling in response to alternating glacial and interglacial conditions, the land at Point Lobos was slowly rising in response to stresses imposed by the plate motions. As it rose the old wave-cut platforms became elevated above the sea in the form of marine terraces (Fig. 38). In an area of continuing uplift, the oldest terraces are the highest and generally the most poorly preserved. Figure 39 shows the uplifted marine terraces seen from the southeastern side of Moss Cove. The shoreline scar associated with the third terrace is visible as a faint horizontal trace on a distant hillside. INTERGLACIAL EPISODE GLACIAL EPISODE Figure 36. During the late Pleistocene Epoch, sea level rose and fell dramatically as the world alternated between glacial and interglacial episodes. Figure 37. If sea level remains more or less constant for a long (thousands of years) period of time, the sea cuts a platform along the shoreline. WAVE-CUT PLATFORM 17 1. EROSION OF A WAVE-CUT PLATFORM UPLIFT 2. SEA-LEVEL FALL 3. SEA-LEVEL RISE, NEW PLATFORM CUT UPLIFT UPLIFT 4. SEA-LEVEL FALL ELEVATED MARINE TERRACES Figure 38. Elevated marine terraces result where sea level fluctuates significantly on at uplifting coast. When sea level remains more or less constant during a high stand of the sea, the waves cut a platform into the shoreline. If sea level then falls this platform is exposed. By the time sea level returns to it approximate previous position, the wave-cut terrace has been elevated to a position above the sea. Repetition of these processed create a set of elevated marine terraces that form “stair steps” away from the sea 18 HIGHEST (OLDEST) MIDDLE LOWEST (YOUNGEST) SHORELINE SCAR ASSOCIATED WITH THE HIGHEST TERRACE Figure 39. Elevated marine terraces on the northwest side of Point Lobos (Moss Cove in the foreground). Three terraces are visible. The lowest (and youngest) terrace is very well developed at the margin of Moss Cove. A second terrace is visible near the highway; Hudson House is built on it. A third terrace can be seen in the distance. The shoreline scar associated with this terraces is visible on the distant hillside. 19 When did Point Lobos materialize as a physical feature? The configuration of the Pleistocene terraces give us a clue. Figure 40 shows the likely shoreline associated with the middle terrace which formed between 120,000 and 200,000 years ago. The headland of Point Lobos did not exist during the development of the middle terrace , although the higher parts of the Reserve might have projected as islands off a small headland developed at the present site of Rat Hill. In a reconstruction of the Reserve’s shoreline during the last high-stand of the sea some 100,000 years ago, based on the configuration of the lowest terrace, the headland of Point Lobos is there and an older, larger version of Whalers Cove exists (Fig. 41). Terrace distributions drawn from (Clark et al., 1997). Rat Hill PACIFIC OCEAN APPROXIMATE SHORELINE of the middle terrace Dashed line: present-day shoreline 0.5 mi 1 km Figure 40. Inferred shoreline at the time of the middle marine terrace development (between 120,000 and 200,000 years ago). Shoreline is based on water depths inferred from the terrace position and current elevations. APPROXIMATE SHORELINE of the lowest terrace Dashed line: present-day Point Lobos shoreline PACIFIC OCEAN 0.5 mi 1 km Figure 41. Inferred shoreline at the time of the lowest marine terrace development about 100,000 years ago. 20 The shoreline continues to change. Large winter storm waves actively erode the coast along the south shore of the Reserve (Fig. 42). It seems likely that a few thousand years hence the coast there will be markedly different. Figure 43 shows the broader geological aspects of the central California coast geology today. The Salinia terrane is interpreted as a southern extension of the Sierra Nevada batholith that was torn off the North American continent and driven north with the Pacific Plate. The movement continues today. Large active faults continue to reflect the movement along the boundary separating the Pacific and North American Plates (Fig. 44). Figure 42. Currently eroding shoreline, Weston Beach. ? SAF SN SIERRA NEVADA BATHOLITH SA SALINIA TERRANE SA OCEANIC CRUST SN SAF - San Andreas Fault M M - Monterey SA SB - Santa Barbara - Monterey Peninsula Faults SB SAF PACIFIC OCEAN 21 Figure 43. Major geologic elements of California’s Central Coast. The Salinia terrane is considered by most geologists today to represent a southern extension of the Sierra Nevada Batholith, that because it lay on the seaward side of the San Andreas Fault, was transported to the northwest as part of the Pacific Plate. The great faults of Central California will continue to displace these rocks. Point Lobos will continue to shift to the northwest. Some geologists feel that rocks originally connected to those at Point Lobos have been carried as far north as Point Reyes by the combined slippage along the San Gregorio and San Andreas Faults. The vertical forces associated with stresses of the plate boundary will continue to warp the coastline. Powerful storm waves will continue to assault the shoreline, continuously remolding it. In the geologic future, Point Lobos, as we know it today will cease to exist. We are most fortunate to experience its present-day magnificence; like all things on earth, it will not last forever. NORTH AMERICAN PLATE PACIFIC PLATE Figure 44. Major faults in the Monterey Bay area. Their displacements reflect the interaction of the great crustal plates that created them. 22 About the Paleogeographic Reconstructions The Paleogeographic reconstructions shown here are based on a combination of several sources: Atwaterʼs illuminating animations of the tectonic development of the California Coast (http://emvc.geol.ucsb.edu/forteachers/CA_geology.htm), Blakelyʼs beautiful reconstructions of the western half of the continent and the Colorado Plateau through the past hundred million years (http:://www2.nau.edu), and a host of regional reconstructions, e. g., M. S. Clark, San Joaquin Valley Geology (http://www.sjvgeology.com), all influenced by 50+ years of study of selected sedimentary rocks in the central and southern California Coast Ranges. However, the detailed configurations of ancient shorelines shown here are mostly fanciful and should be considered only as possible examples of how this region might have looked at different times in the geologic past. References Burnham, K., 2005, Point Lobos to Point Reyes: Evidence of ± 180 km offset on the San Gregorio and northern San Andreas Faults: in C. Stevens and J. Cooper, eds., Cenozoic Deformation in the central Coast Ranges, California, Field Trip Guidebook, Pacific Section SEPM (Society for Sedimentary Geology), p. 1-29. Clark, J. C., 1997, Neotectonics of the San Gregorio fault zone: age dating controls on offset history and slip rates: U. S. Geological Survey Final Technical Report, 30 p. Clark. J. C., Dupré, W.R. and Rosenberg, L.I., 1997, Geologic map of the Monterey and Seaside 7.5−minute quadrangles, Monterey County, California: a digital database: plot derived from U. S. Geological Survey Open−file report 97−30. Bowen, O. E., 1965, Point Lobos, a Geologic Guide: California Division of Mines and Geology, Mineral Information Service, v. 18, No. 4, p. 60-67. Kistler, R. W., and Champion, D. E., 2001, U. S. Geological Survey Open-File Report 01-453). Nili-Esfahani, A., 1965, Investigation of Paleocene strata, Point Lobos, Monterey County, California [M.A. thesis]: Los Angeles, University of California, 228 p. Compiled by Ed Clifton, August, 2016 23