Field Report about the Geology of Kota Keluarga, Laiya,San Juan, Batangas, Philippines

August 19, 2017 | Author: Rene Villeneuve | Category: Clastic Rock, Minerals, Rock (Geology), Limestone, Petrology
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The report answers some questions that is provided by our professor in the course, Paleontology....



SUBMITTED BY: Noel Patrick L. Soberano SUBMITTED TO: Prof. Marissa P. Cerezo

1. Describe the structures and textures of the outcrops found in Station 1 in terms of color of rocks (weathered and fresh surface), mineralogical composition, fossil content (description), outcrop extent (height and width), types of sedimentary rock. STATION 1- Outcrop 1 Station 1 is located in an elevated portion of the resort. The first area in station 1 is a site of boulders having a size range of 0.5 to 0.75 m in diameter based on a photograph scale (Fig 1.1). The width of the area ranges from 15 to 30 m while the altitude is measured in an altimeter as 75 m above sea level. Escarpments measuring around 1.5 to 3.0 m high are also observed (Fig 1.1). A steep edge is facing the sea where the next outcrop is situated(Fig 1.2). The rocks in the area are mostly white to creamy white. A sample rock effervesces as 0.1 M (diluted) HCl is dropped on its surface. Thus the rock is composed mostly of Carbonate minerals such as Calcite or Dolomite. These white rocks are formed from organisms that grow in place , coral reefs in this case. These rocks are then classified as limestones. Boundstone is a more specific term for these kinds of rocks. Because these rocks are once a natural habitat for marine organisms, fossil organisms are found in the rocks. Trace fossils are mostly seen in the area such as molds and casts of shells (Fig 1.3), minute gastropods are also found and could be mistaken for ooids due to their somewhat spherical shape (Fig. 1.4).

Figure 1.1 Boulders are scattered and an scarp is exposed in the middle left of the image.

Figure 1.2 The steep edge of the first area facing towards the sea.

Figure 1.3 An impression of a shellfish in side view

Figure 1.4 The left portion of the coin shows minute sphere like structures.

STATION 1- Outcrop 2 The next area is a roadcut at a lower elevation with respect to the first outcrop which is 75 m high. The elevation of this next outcrop is around 40 to 60 m above sea level. This outcrop exposes a rock face with bedding structures. Two lithologies are observed in the area : Limestone and conglomerate. The white rocks that are seen in the upper outcrop (outcrop 1) are of similar appearance to the rocks in this outcrop just below it. The clasts and matrix of the conglomerate was not well analyzed due to time constraints and obstacles (Fig 1.5). The outcrop is around 5 to 8 m high with respect to the north-east inclining road. The conglomerate bed along with a carbonate bed underlying it is tilted at an angle with the horizontal and is then truncated by a slightly horizontal upper carbonate rocks. This structure is exhibiting a type of unconformity known as an angular unconformity.

Figure 1.5 The exposed white rock faces are of similar characteristics from outcrop 1. The tilted conglomerate beds are partially obscured by the bushes in this image

2. Describe Station 2 in the same manner as Station 1. STATION 2 – Outcrop 1 Station 2 is situated at the rock exposures bordered by seawater. The first outcrop is around 20 to 25 m high and 25 to 30 m wide (Fig 2.1). It faces South-East in reference to the map. It shows grey-colored rocks. By judging through its color, the rock is intermediate in composition. The texture of the rock is porphyritic because dark mineral crystals, which are termed phenocrysts, are embedded on a fine-grained groundmass (Fig 2.2). Minerals observed in hand specimen include pyroxene, amphiboles, plagioclase, and some bitotites. The cleavages of the dark elongated minerals are carefully observed under a 10x hand lens (Fig 2.3) and a 30 to 50x magnifying lens. Some of the dark minerals show approximately 90-90 cleavage while others show 120-60. Thus this gives us an idea that amphibole and pyroxene are present in the rock specimen. Biotite is easily distinguished due to its flaky appearance and hence its one directional cleavage. The white crystals are identified as plagioclase feldspars and not quartz because quartz exhibit a conchoidal fracture and is not associated with the formation of pyroxenes in the bowen’s reaction series. Another distinguishing characteristic of plagioclase is its striations, though this feature is not clearly seen in the rock specimen under a hand lens. The rock in this station is mostly igneous and it can be identified as porphyritic andesite. The rock outcrop also shows conspicuous features such as joints which extends 10 to 13 m in length. The joints in the rock face facing the sea are inclined at an angle. There are about 5 to 6 major joints in the area each with subparallel or nonparallel orientation with each other (Fig 2.1). Another distinct joint feature evident in this area is the columnar joints, that are only exposed on the lower and middle left portion of the outcrop (Fig 2.4).

Figure 2.1 The extent of the rock outcrop in station 2 is measured using the height of the person nearest to the latter. The joints are also measured.

Figure 2.2 The rock sample from station 2

Figure 2.3 The rock specimen in Fig 2.2 magnified by a lens of a low magnification.

Figure 2.4 Columnar jointing is observed in the middle left portion of the image. This portion of the outcrop faces south east in reference to the topographic map.

3. Have you plotted your location on the map? Discuss the procedures you observed in determining the location of both stations on the map. Without the use of advanced technological devices such as GPS, our group were able to locate our position in station 1 and station 2 with the aid of a topographic map, brunton compass, rulers and protractors. In the first station our location is still undeterminable in the map because no distinct topographic structure can be used as a mark for triangulation. Though with the aid of an altimeter our location is known to be elevated above the shore at 75 m, we highlighted all the possible areas having dense contour lines at a distance 60 to 75 m from the coastline. The location is still not yet determinable with the given information above. Thus the location of station 1 is set aside and to be analyzed for later. The land features in the second station allowed us to relate the surrounding features with that in the topographic map. The bearing of three points: Loco Loco point, Malagundi point, and the Punas point, are used as markers. The Loco Loco point is the point we suspect as the one nearest to our location. The other two points are the further points in reference to our line of sight. The intersection of the bearing lines of the above mentioned points indicates the location of station 2 in the map and hence our location (Fig 3.1). The intersection point is then plotted on the map and the coordinates are determined as 121˚25’55’’(latitude) and 13˚39’43’’(longitude). This is the coordinates of the last destination in Station 2. And as for the coordinates of station 1, an estimation of the distance travelled from station 1 to station 2 is used and measured in map distance. The earlier highlighted area with dense contours was compared to the actual location of station 2 and the coordinates of station 1 is then estimated to be 121˚26’ (latitude) and 13˚40’ (longitude).

Figure 3.1 The colored lines are the bearings of the three points.

4. Have you determined the general attitude of joints in Station 2? What does this kind of jointing imply? How do you measure strike and dip values, illustrate. Large joints are present in station 2, but there are also joints that exhibit elongated polygonal columns in some faces of the outcrop . The attitude of the columns varies in each exposure. The plunge of the joints facing south east is determined to be in the southwest direction (Fig 2.4). There are areas wherein the columnar joints are well developed while there are also faces in the outcrop that shows the latter jointing which are not so well developed (Fig 2.4). The underdeveloped columnar joints are formed by nonuniform thermal gradients and contraction processes that is if a magma is still moving slightly as the joints form. However the well developed polygonal prismatic columnar joints (Fig 4.1) are due to the nearly perfect uniformity of the cooling rate, thermal gradient, and stress field of a cooling magma body (Hatcher,1995). These kind of joints are nontectonic. The other larger joints on the other hand may be formed due to tectonic stresses or hydraulic fracturing.

Figure 4.1 Columnar joints exposed at a high elevation in the rock outcrop

5. What is the major structure seen between the limestone and clastics in Station 1? Describe in terms of attitude, lithologic boundary and type and your basis of identification. The structure seen in station 1 is an unconformity. At first the structure is thought to be massive, but looking at it carefully would reveal beds of clastic rocks. These clastic rocks are identified as conglomerates. The conglomerate bed is tilted at an angle and is then truncated by the coralline limestone above it. Thus this unconformity is an angular unconformity. The outcrop faces South East and the bed is dipping in the Southwest direction.

6. Describe the shape, composition and size of clasts of sedimentary rock underlying the limestone in Station 1. The clasts size of the conglomerate ranges from pebble to cobble. Its composition is mostly composed of feldspars, and rocks of intermediate color, which could be some of the fragments of the andesitic rocks in station 2. The clasts are rounded to well rounded with a low sphericity.

7. Make an assumption on the relative age of rocks in Stations 1 and 2. Explain. The rocks in station 1 are limestones and conglomerate. And the rocks in the next station are identified as andesite. Some of the clasts of the conglomerate is similar to the andesite in station 2. The law of components could be used to analyze the relative ages of the two rocks. It states that if clasts are found in a formation then the clasts must be older than the formation. The andesite in station 2 must be older than the conglomerate in station 1.

8. What is your analysis on the presence of limestone in Station 2? Station 2 could be at one time submerged in water and coral reefs formed which now represent the limestone rocks in the latter station. Eventually the outcrop in station 2 is then exposed by the lowering of the sea level or by the uplift of the area.

9. What are the minerals found in the area ( in both stations). List with corresponding host rock. STATION 1:  Organic Calcium Carbonate (Calcite) – limestone (boundstone)

STATION 2:  Pyroxene – Andesite  Hornblende – Andesite  Plagioclase – Andesite

10. Describe the beach sand. Are they in situ or inherent in the area? Explain your analysis. When viewed under a 10x to 30x hand lens, the loose sand sample contains grains that are subangular to rounded in roundness. The sorting of the grains is moderately sorted to poorly sorted ( Fig 10.1). The beach sand also contains fossils (Fig 10.2 to Fig 10.4). The fossils are mostly foraminiferas and microscopic shell organisms (gastropods). Some microfossils exhibit spiky surfaces. From the characteristics of the sand grains and microfossils , we can say that these fragments have not travelled far because the grains are still angular and some microfossils have spikes. Thus the beach sand is inherent in the area.

Figure 10.1 The beach sand sample which is composed of granular rock fragments, broken glass fragments, and microfossils

Figure 10.2 Round microfossils distinct holes and pores

Figure 10.3 Microfossils with symmetrical spines

Figure 10.4 Sand sized shells [email protected]

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