Geology 122 (Principles of Geomorphology)

FIELD TRIP REPORT Ilocos Norte, Philippines

John Michael Austria Jeffrey Batolbatol Dominic Bornilla

John Dale Dianala Karizz Anne Morante

Kenny Lyne Tabayocyoc Mark Anthony Vibas

I. Introduction

Geomorphological analysis is an important tool in field mapping as it provides means to delineate the geology of areas that are not visited first-hand. With the aid of topographic maps, satellite images, aerial photos, and others, observations made at a distance give a scientific basis for delineating features such as faults, lineaments, contacts, and rock composition.

Objective The objective of this field trip is to expose Geology 122 students AY 2010-2011 to actual

examples of landforms and to exercise observational skills in identifying these.

II. Methodology

The fieldtrip was conducted in seven different locations in Ilocos Norte on September 25, 2010, mainly: (1) Bacarra Bridge, (2) Bojeador Lighthouse, (3) Capurpurauan, (4) Bangui Windmills Viewdeck, (5) Bulu River, Dumalneg, (6) near Hannah Beach Resort, Pagudpud, and (7) Nagsurot, Burgos. Observations of landforms were done remotely, and compared to features observed in topographic maps and Shuttle Radar Topography Mission (SRTM) images. A 1:250,000 scale NAMRIA (1992) topographic map of the Laoag Quadrangle was used as a base map for locating with the aid of a GPS receiver.

III. Results and Discussion

Around 80 kilometers were covered in the whole field trip, from the towns of Bacarra to Dumalneg to the municipality of Pagudpud overlooking Pasaleng Bay. The locations of the stops made in the field trip are shown in Figure 1.

A. Bacarra Bridge – 18°14’49.4”N 120°37’17.4”E The Bacarra Bridge is found over the Vintar River, which drains towards the South

China Sea. The river appears moderately sinuous and is about twenty meters wide. Sand to gravel-sized, poorly sorted, polymictic, rounded sediments are found along the flood banks of the river. Retaining walls are found along the sides of the banks.

At the background, a densely vegetated mountain ridge is present (Figure 2A). The slope of this ridge was observed to be less than 30°, suggesting that it is probably a cuesta. The high relief of the mountains could mean that they are composed of resistant beds. The presence of triangular facets suggests the presence of a fault.

Figure 1. Location map of stops in the fieldtrip. 1-Bacarra Bridge, 2-Bojeador Lighthouse, 3-Capurpurauan Coast, 4-Bangui Windmills Viewdeck, 5-Bulu River, 6-near Hannah’s Beach Resort.

The NE-SW trending Vigan-Aggao Fault, a wrench fault splay of the Philippine fault, is said to pass through this area (Pinet and Stephan, 1990). This is expressed by the topography as seen in the topographic map and the SRTM image (Figure 2 B and C). It can be noted that the Vintar River follows this zone of weakness.

B. Bojeador Lighthouse - 18°30’50.”N 120°35’44.6”E The Bojeador Lighthouse overlooks the western coast of Ilocos Norte. The shoreline

is considered emergent with the presence of uplifted coralline limestones above water on the coast (Figure 3). Previous studies (Geology 170 [unpublished], 2010) have found that the shoreline is comprised mainly of limestone. Limestone hills with typical concave slopes is also present near the area.

The lighthouse is built on top of volcanics (Geology 170 [unpublished], 2010). Highly vegetated hills with a “windgap” are found landward (Figure 4). The topography appears irregular with moderate relief (Figure 5), characteristic of volcanic terrains.

Figure 2. A) The Vintar River with the mountain ridge at the background. Photo towards northeast. B) Topographic map and C) SRTM images show relief of the area and a possible fault (black and red lines). The stop is marked in orange.

A)

B) C)

A)

C. Capurpurauan - 18°32’19.4”N 120°38’54.9”E At least two different lithologies are present in the Capurpurauan coast area:

Limestones that are calcarenitic in nature and mafic volcanics. (Figure 5) The limestones form thickly vegetated ridges with concave slopes. A particular ridge near the Capurpurauan view deck is recognized to be a cuesta (Figure 6). The limestones are found on the east side of the coast, including the famous White Rock Formation of the municipality. This particular

Figure 3. Uplifted coralline limestones (outlined in red) on the coast are characteristic of an emergent coastline. Photo towards southwest.

Figure 4. Heavily vegetated hills with a "windgap." Photo towards east.

Figure 5. Stops marked in the topographic map and SRTM image. 2 (pink) - Burgos Lighthouse; 3 (yellow) – Capurpurauan; 4 (white) - Bangui Windmills Viewdeck; 7 (green) - Nagsurot, Burgos.

limestone is a sea stack, formed by erosion of existing rock through wave action and tidal fluctuation (Figure 8).

The volcanics, on the other hand, are found at the western portion of the coast, with a thinly vegetated rolling topography. It comprises tall cliffs about 25-30 meters high and most of the coastal bedrock. A tunnel found at the base of the cliffs is thought to be as a lava tube (Figure 7). These are formed when the rim of the lava channel solidifies first, and the lava continues to flow until it is emptied, leaving behind the hollow tube.

It is deduced that the volcanics in the area are part of the lava front that rammed into the older limestones. This is evidenced by its higher elevation topography and the presence of bleached volcanic rocks near the contact.

.

Figure 6. A limestone cuesta. Photo towards northeast.

Figure 7. A "lava tube" (in circle) found at the base of the cliffs made of volcanics. Photo towards west.

Figure 8. Sea stacks of limestone.

D. Bangui Windmills Viewdeck - 18°31'15.83"N 120°41'51.37"E From the Bangui View deck, a lot of different lithologies and geomorphologic features are

seen. Looking east (Figure 7), plains with rice fields and the front of the Ilocos mountains are seen. The lithologies comprising these mountains are clastics sedimentary rocks such as sandstone, siltstone and claystone. These mountains have thick vegatation with some grassland patches. Planar slopes with rails and gullies are also observed in these mountains, suggesting numerous sites of erosion by streams (Figure 8). At the lower right of these mountains, limestone hills with the typical lush and dense vegetation of limestone are observed. Also seen in this view is the almost straight Baruyen river draining to the Bangui Bay. This river provides irrigation for the rice fields in the vicinity.

Figure 11. Photo showing the coast, the wave-dominated delta, sand dunes and the peridotite and schist ridges. Photo towards northeast

Figure 9. Topographic map showing the view of the Photo.

limestone

clastics

Figure 9. Topographic map showing the view of the Photo.

Figure 10. Eastern view from the Bangui Windmills view deck showing the clastic mountains with rails (red lines) and gullies (blue arrows), the limestone hills and the

Baruyen river in the rice fields.

Looking north, the view of the Bangui delta with the exquisite windmills is seen. Sparsely vegetated ridges composed of peridotites and schists are present in the western part of the bay. The coast is observed to be emergent. A wave-dominated delta is also present in the area as characterized by its lobate shape. Sand dunes are also seen on the eastern part of the coast.

Schists &

peridotite

sand dunes

plains

rails

gullies

delta

E. Bulu River, Dumalneg - 18°31'51.0"N 120°50'2.8"E

Exposed along the Bulu river in the town of Dumalneg are highly indurated sedimentary

units (Figure 13) composed of sandstones, conglomerates, breccias, cherts and shales. The area

is observed to abruptly change from a relatively flat topography to having steep slopes forming

mountain. This is indicative of faulting. Fault traces are shown in Figure 12B. Further evidence

include the presence of triangular facets and the observation of a fault scarp (Figure 13 and 14).

The Bulu river, which is considered to be meandering, cuts across the Patapat

mountains. The river is dominated by rounded cobbles to boulder-sized igneous and

sedimentary rocks and river terraces are present.

Figure 12. A) Topographic map showing the relief and marking the location of the stops: 5(blue) –Bulu River, Dumalneg, 6(red) – Hannah beach, Pagudpudthe relief of the area. B) SRTM image showing the traces of the inferred faults: Bangui fault

in yellow, Viggan-Aggao in orange.

Stops marked in the topographic map and SRTM image. 2 (pink) - Burgos Lighthouse; 3 (yellow) – Capurpurauan; 4 (white) - Bangui Windmills Viewdeck; 7 (green) - Nagsurot, Burgos.

A B

Figure 13. Photograph showing the sedimentary units, the Bulu River that cuts the units, the river terraces, and the fault scarp (bigger photo of the scarp shown on Fig. 14). The photo is towards Northwest.

Sedimentary units

Bulu river

fault scarp

River Terraces

F. Hannah Beach Resort, Pagudpud - 18°37'24.5"N 120°51'30.3"E

Hannah Beach is located at the mouth of Baugan bay. The beach is composed of well-

sorted, buff-colored sand and skeletal fragments of foraminifera and other marine organisms.

The coastline is emergent as evidenced by the abrupt change in elevation near the coast, the

presence of terraces as well as some corals on the headland. Looking north, sea stacks

composed of peridotites could be seen. Southwards, ridges composed of interbeds of

fossiliferous sandstone, conglomerates and tuffaceous sandstones are observed. A sea arch

made up of the same lithology is seen on the way to the beach.

Figure 14. Photograph showing a fault scarp, characterized by its steep, sharp planar feature and sparse vegetation.

It is a scarp of a normal fault, with the movement of the blocks annotated in the figure.

Figure 15. Photograph of Hannah beach showing the peridotite sea stacks and the sedimentary interbeds ridges.

The photo is towards northeast.

sea stacks sedimentary interbeds

F. Nagsurot, Burgos - 18°30’56.63"N 120°37'47.71"E

Limestone ridges that exhibit karst topography are observed in Nagsurot, Burgos (Figure

16). Limestones can easily be recognized even from a far because of their distinctive dense

vegetation. Many plants and even trees, including Molave, grow in limestone soils because

these soils are able to hold much water. This contrasts very much to ophiolitic ridges, wherein

the topography is rolling and not much trees is found, only grasses and shrubs. This can be

attributed to the crystalline nature of igneous rocks, making them more difficult to weather

than the limestones.

IV. Conclusions

Field mapping, field observations and interpretations are made easier and less complex

through the understanding of geomorphology. One need not go to an outcrop far away and

take a sample of it to be able to infer its lithology and note its structures. With a map, good

observational skills and some basic geomorphology concepts, you’re good to go.

Figure 16. Stops marked in the topographic map and SRTM image. 2 (pink) - Burgos Lighthouse; 3 (yellow) – Capurpurauan; 4 (white) - Bangui Windmills Viewdeck; 7 (green) - Nagsurot, Burgos. (same as Figure 5)