ANALYSIS OF THE STRATIGRAPHY OF CHOSEN AREAS

WITHIN THE ILOCOS REGION

A Paper Submitted to

Emmanuel A. Codillo

Joselito P. Duyanen, PhD.

Allan Gil S. Fernando, Ph. D.

Jose Dominick S. Guballa

National Institute of Geological Sciences

In Partial Fulfillment of the Course Requirements in

Geology 120 Principles of Stratigraphy

Submitted by

PEALE AXEL P. BONDOC

RUTH ESTHER G. DELINA

ADRIENNE NICOLE S. FERNANDEZ

FRANK PERRY T. RUBIA

1

TABLE OF CONTENTS

Page

INTRODUCTION…………………………………………………………….………..3

RECENT SEDIMENTARY ENVIRONMENTS

La Paz Sand Dunes, Ilocos Norte………………………….………..…….….6

General DescriptionCompositional AnalysisProvenance and InterpretationSheared Zone

Luna Beach Deposits, La Union…………………………………….….…….9General DescriptionProvenanceSedimentary Structures and Processess

MINOR OUTCROPS

Ilocos Norte

Kapurpurawan, Burgos…………………………………………………....…11Outcrop 1

General Description and StratigraphyInterpretation

Outcrop 2General Description and StratigraphyInterpretation

San Nicolas Outcrop…………………………………………………………14General DescriptionInterpretation

Ilocos Sur

Suso Outcrop………………………………………………………………….16General DescriptionLithology and StructuresInterpretation

2

Santiago Outcrop……………………………………………………………..18General Description and StratigraphyInterpretation

Tagudin-Cervantes Outcrop. ………………………………………….……21General DescriptionStratigraphy and StructuresInterpretation

MAJOR OUTCROPS

Solvec Point, Ilocos Sur……………………………………………………...26General DescriptionSection DescriptionGeologic History

San Juan Outcrop, La Union .……………………………………………….28General DescriptionSection DescriptionGeologic History

NCC Quarry, Pangasinan ………………………………………………..…...31

REFERENCES…………………………………………………….........………………..35

APPENDIX…………………………………………………………………....................36

INTRODUCTION

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Stratigraphy is defined by dictionaries as the study of rock strata, its origin,

composition, and development. This discipline of geology is useful for the

study of, but not restricted to, sedimentary and volcanic igneous rocks.

Geology 120 ‘Principles of Stratigraphy’ is one of the geology courses that is

offered by the National Institute of Geological Sciences. According to the

syllabus given by Prof. Fernando and others (2015), the end-objective of the

course is, “to be able to conduct fieldwork independently, and apply basic

principles in geology in analyzing the stratigraphic relationships of rocks.”

One of its major requirements is a fieldwork to test the skills learned and

developed in this class and previously taken geology subjects.

The fieldwork undertaken by this batch is a four day activity, from April 10 to

13, 2015, along the stretch of Ilocos Region. This region—Ilocos Norte, Ilocos

Sur, La Union, and Pangasinan—is located on the northwestern portion of the

Philippines. It is bounded by the South China Sea to the west, Luzon Strait to

the north, Cordillera Mountain Range to the east, and the Central Luzon

Region to the south. Climate in this region is Type I: dry from November to

April and wet for the rest of the year.

The geography of the region is noteworthy to stratigraphy students because

it is a relatively slender coastal region immediately bounded by the highlands

of Cordillera, and sediments from the mountains are dumped in this region.

Furthermore, the climate of the region distinguishes it from other regions, in

that it was able to develop a subaeolian environment. Aurelio and Peña

(2008) describe the Ilocos Region under a sedimentary basin, contiguous

with the Central Luzon plains.

4

Figure 1. The visited areas during the fieldwork, according to Google Earth. Horizontal distance from

Kapurpurawan to Northern Cement Corporation is around 260 kilometers.

The batch was divided into groups for the fieldwork. For every outcrop, each

group describes the outcrop from afar, then takes a closer look and infers a

possible geologic history. Groups were assigned to report their findings with

the class, and afterwards, the professors and instructors will perform a

discussion about the outcrop. The general lithology and stratigraphy of each

outcrop together with its possible depositional environment and geologic

history were discussed in the field.

Ten breath-taking outcrops and two recent sedimentary environments across

the region were visited from the north on the course of the fieldwork. The

visited sites which are shown in Figure 1 will be discussed in this paper.

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Recent Sedimentary Environments

La Paz Sand Dunes

The La Paz Sand Dunes is a sandy coastal desert of approximately 85 square

kilometers facing the coast of the South China Sea. Also known as Bantay

Bimmaboy from the pig-like appearance of the dunes, this area belongs to

the Ilocos Norte Sand Dunes (INSD) which is a forty (40) kilometer narrow

zone of low-lying elongated hills running from the coast of Currimao located

in the south to Pasuquin in the north (see Fig. 2 left).

General Description

The area of study comprises asymmetrical ripples of unconsolidated sand,

with its steep slopes or the lee side dipping toward the south. This implies

the present day southward wind current direction transporting sand sized

sediments, forming these ripples. From the drop off point to the south, a

change from linguoid to sinous, then to straight-crested ripples (refer to Fig.

6

Figure 2. (Left) A map showing the extents of the Ilocos Norte Sand Dunes (INSD), Paoay Lake and the La Paz Sand Dunes. (Right) Section of the Laoag topographic map showing the Laoag River and the La Paz Sand Dunes.

A B C

Figure 3. Different current ripple types found on the La Paz Sand Dunes: linguoid ripples in (A) sinous in (B) and straight crested in (C).

3) was observed suggesting a decrease in flow velocity and level of energy

from the drop off point to the direction of the current.

Compositional Analysis

From the drop off point to the southeast, 3 sets of sand sediments weighing 3

grams each were collected, all of which are well sorted. The sand samples

are composed of sub-angular to rounded quartz, magnetite, lithic and chert

fragments. Further grain analysis was conducted; results are summarized on

the table below.

Sample Bag (arranged

from NW to SE)

Quartz (%)Lithic Fragments

(%) Magnetite (%) Chert (%) Grain size

1 40 30 20 10 Fine sand2 35 30 25 10 Fine - medium sand

3 30 60 *in lithic fragments 10 Medium - coarse

sand

As observed from the ripples, magnetite minerals tend to accumulate at the

stoss side because of its higher specific gravity compared to the other

sediments. Abundance of quartz, magnetite and lithic fragments imply an

igneous origin. Moroever, near the sinuous ripples, a friable unit of cross-

laminated black and white layers of sand was observed. The thinly bedded

black laminae were mainly composed of magnetite while the white layers

consist of quartz and lithic fragments. This apparent cross section further

proves the igneous nature of the sediments.

Provenance and Interpretation

This igneous origin is supported by the geologic evolution of the INSD

according to the studies of (JSP, 2008). It traces back to the sediments

deposited from rivers such as Quiaoit, Laoag (as seen Fig. 2 Right), Bacarra,

and Pasuquin Rivers which were then transported seaward by the strong

longshore currents of the South China Sea in convergence with the Pacific

Ocean. These sediments were then concentrated in shallow nearshore

environments and the high sediment supply and strong wave action in these

areas enabled the formation of sandbars. These sandbars then migrated to

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Table 1. Grain analysis of the obtained samples

the beach environment because of incessant wave action. As these were

exposed and dried up, wind currents acted upon the unconsolidated sand

sediments.

Landward-migration of the sand dunes due to incessant wind and wave

accession caused the clogging of a pre-existing embayment creating the

Paoay Lake. This is further proven by the planar outline of its western border

due to the movement of the sediments by and parallel to the direction of the

longshore currents. Continued wind action produced larger and elongated

mounds along the beach zone forming the first line of sand dunes of the INSD

(JSP, 2008)

Sheared Zone

A fifteen (15) meter high and forty (40) meter wide outcrop located at 18°

12.533’ N, 120° 32.208’’ was found few meters southeast the drop off point.

It may have been another source of the sand deposits as suggested by the

coarsening grain size towards the direction of this unit. Labeled as unit 3, the

bottommost bed (Fig. 5A) is composed of serpentinized peridotites and

mylonite and is separated to unit 2 by a friable reddish layer formed from the

oxidation of the peridotites. Unit 2 is composed of highly weathered

calcareous rocks and is overlain by conglomerates (unit 1). Moreover, a unit

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http://adventuroj.com/2012/12/01/paoay-tour-paoay-church-malacanang-of-the-north-and-the-paoay-lake/

2

3

Figure 4. (Left) A section of the Laoag topographic map from Namria showing the Paoay Lake (Right) Eastern portion of the lake

composed of chert (Fig. 5D) was found in the southernmost portion of the

outcrop (not seen in the picture).

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Figure 5. (A) The outcrop showing three different units, (B) serpentized peridotite sample, (C) mylonites, and (D) bedded chert. Height of scale: 1.5 meters (scale not on base of outcrop)

A D

2

3

However, no observed contact with the adjacent unit was found. The

different lithologies and obscure contacts between units characterize a

mélange, however, extensive studies are required for it to be called one.

Instead, this chaotic mix of deposit is said to be situated in a sheared zone,

which is supported by the mylonites found in unit 3. This further implies the

ductile deformation of the source rocks through a large shear strain induced

by a fault. A rotational, non co-axial component can be induced by the

shearing motion which only preserves remnants of the primary rock (Fossen,

2010).

Luna Beach

General Description

Situated at the Bangar Quadrangle in La Union, Luna Beach (16˚50’48.9”N,

120˚20’34.4”E) is a transitional beach environment dominated by gravel

sized grains. These include rounded to well-rounded grains of diorite,

porphyritic andesite, low-grade metamorphic rocks, sandstones, and shell

and coral bioclasts. As observed from the area, the sediments range from

pebble to cobble in size and bladed to oblate in shape. These observations

were further supported by the results obtained from the Geology 150 Grain

Morphology exercise.

Possible Provenance

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SE SEA

Figure 6. (Left) A section of the Bangar topographic map from Namria showing the Amburayan River and Luna Beach (in red dots), (Right) Pebble imbrication emphasized by red lines, Photo by Abby Villaruel

Shell and coral bioclasts are derived from the nearby reef while the igneous

rocks, comprising majority of the beach deposits, are possibly from the

Cordillera Mountain Range. As seen on the map (Fig. 6 Left) these sediments

are brought down by the Amburayan River which originates from Cordillera

Mountains and traverses the provinces of Benguet, La Union and Ilocos Sur. It

empties to the South China Sea and feeds the Luna Beach with sediments

through the strong long shore currents. The pebbly beach reflects the high

competence and capacity of the Amburayan River needed to transport these

gravel sized sediments. It was also discussed that these sediments were a

product of delta switching, however, the sediments are too coarse for a delta.

Sedimentary Structures and Processes

The roundness and oblate nature of majority of the grains reflect the

influence of the swash back swash of the waves from the sea after these

rocks were deposited to the coastline. Due to the strong wave action, these

pebbles are oriented in their most stable position, resulting to a preferred

orientation called imbrication. The pebbles and cobbles of the Luna Beach

dip toward the sea. This imbrication (Fig. 6 Right) is observed in the berm,

the highest part of the beach where the coarsest sediments are located and

is covered by water during high tides.

Minor Outcrops

Kapurpurawan, Burgos, Ilocos Norte

Outcrop 1

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Entablature

Colonnade

General Description and Stratigraphy

The outcrop is located along the road to the famous Kapurpurawan rock

formation. Specifically located at 18°31.732’ N, 120°38.384’ E, the observed

outcrop is 50 meters wide and 7.5 meters high. From afar, it has been

observed to consist of three units separated by conformable contacts. Upon

closer inspection, the bottom unit is identified as a volcanic breccia with

clasts of porphyritic basalt and altered minerals such as opal and palagonite.

The hard and fractured middle unit is composed of vesicular andesite

occasionally with red bands indicating oxidation. Lastly, the topmost unit is

composed of porphyritic andesite with vesicular andesite inclusions, most

probably from the middle unit. As shown in Fig. 7, the third layer is

characterized by columnar joints generally trending N 50° E, 48° SE.

Interpretation

The bottommost unit is further identified as a hyloclastite which are hydrated

tuff-like breccias produced by phreatomagmatic eruptions associated with

magma-water interaction. Evidence for this claim include the unit’s

autoclastic nature and the presence of opal, a hydrated silica, and an altered

volcanic glass by quenching in water known as palagonite. This places the

unit in a subaqueous setting. The middle layer is believed to be a more

altered unit as compared to the hyaloclastite.

The top layer was formed when andesitic lava flow/s overlain the two layers.

Columnar joints with defined stout cooling joints or colonnade from the base

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. Outcrop along the road to Kapurpurawan. Note the evident cooling joints. Scale: 1.7

to middle and thinner and less defined joints on top called entablature,

indicate thick lava flows. These joints indicate the absence of water influence

since these forms from the thermal contraction within the lava flow/s. This

places the unit in a subaerial environment. In conclusion, the outcrop was

formed from at least two lava flows one of which is formed in a subaqueous

setting and the latter in a subearial environment.

Outcrop 2

General Description and Stratigraphy

Located near the viewdeck, with coordinates 18° 32.273’ N, 120° 39.693’ E,

the black-and-white outcrop (Fig. 8) is around 15 meters wide and about 7

meters high. The rock is composed of fine- to medium-sized sand grains

mainly made up of bioclasts. By the Grabau limestone classification, the

thinly bedded rocks are identified as calcarenite.

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Figure 8. (Left) Black-and-white limestone unit with red lines showing highlighted (Right) Close-up photo of the beds emphasized by the red lines

Mainly white in color, the Kapurpurawan Rock Formation (Fig. 9 Left) is

believed to have the same lithology as the previously discussed limestone

unit. At the base of the outcrop is an uplifted reef. Compared to the top of

the unit, it is dark colored and appears to be porous due to weathering and

dissolution by wave action. It is most likely here that new limestone will form.

The famous Kapurpurawan structure called as yardang is made by wind

action eroding the different sides of the rock, leaving distinct traces on them.

Analysis of thin sections from the outcrop provided by the Nannoworks

Laboratory shows that the rocks of this famous rock formation are

packstones, according to the Dunham classification. It mostly contains

planktonic foraminifera set in a calcite matrix which indicates deposition on a

deep see environment.

Both outcrops are formed from a deep sea environment, specifically on

shelfal regions, evidently supported by the limestone being thinly bedded

rather than massive. It is also supported by the analysis in thin section.

Interpretation

Two outcrops of different rock types were observed namely igneous rocks at

higher elevations overlying the sedimentary units below. Earlier observations

14

Figure 9. (Left) Kapurpurawan Rock Formation (Right) Thin section (50x magnification) of an obtained sample from the KPR provided by the Nannoworks Laboratory

by other researchers put this area under the Bojeador Formation, which

includes lithologies such as conglomerate, graywacke, shale, limestone,

volcanic flows and pyroclastics (Peña, 2008).

The first outcrop shows that it was once in an aquatic to subaquatic

environment and then was exposed and brought into a subaerial

environment, while the second is formed at deep sea environments

particularly in the shelfal areas. Using the law of superposition, assuming no

overturning has occurred, the limestone unit is older than that of the first

outcrop. Columnar joints of the igneous unit, indicative of the top of the bed,

further support the claim.

San Nicolas Outcop

Figure 10. The outcrop showing the contact between marine (MD) and fluvial (FD) deposits. Height of scale: 1.65meters (right)

General description

San Nicolas outcrop is situated in a quarry area in Brgy. San Agustin,

San Nicolas, Ilocos Norte. It has a height of about 12 meters (Fig. 10). From

afar, the outcrop is generally light brown in the upper portion and alternating

light and dark layers in the lower portion displaced by fault sets, dominantly

normal faults. Upon closer inspection, it was observed that the bottom unit is

composed of interbeds of thickly to medium bedded fine and medium

sandstone which suggests periods of low and high energy of deposition.

Fairly well-sorted, sub rounded to rounded grains of thickly bedded fine

15

sandstone have found to have leaf fossil, borings and shell fragments. While

the thickly bedded medium sandstone consists of sub angular to sub rounded

grains of lithic fragments, and trace fossils, specifically burrows. Bioclasts of

preserved fossils such as echinoid spines, corals and forams were also

dispersed throughout the bed indicating that these beds deposition in a

shallow marine environment, specifically on the shelf margin.

The top unit is composed of polymictic conglomerates with clast size

range of cobble to occasional boulder size which is sub rounded to rounded.

Clasts from the outcrop are mostly diorite, porphyritic basalt, mudstone, and

magnetite held together by silt to very fine sand size matrix.

Interpretation

A Channel scour filled with conglomerate with sandstone matrix were also

observed

between

the contact

of marine

sandstone deposits and conglomerate body, indicating an erosional event

and an unconformity. The concave side of the channel points upward

indicating that the

16

Figure 11. The uppermost portion of the outcrop emphasizing the channel scour. Scale: 3m

As shown in Fig. 11, a channel scour observed between the marine

sandstone deposits and conglomerate body indicates an erosional event

forming this disconformity. The concave side of the channel pointing upward

implies that the conglomerate is younger than the sandstone interbeds. The

sharp change in lithology indicates a change in depositional environment.

The interbedded sandstone unit formed in a marine environment is uplifted

by local tectonics to a deltaic environment where the gravel sized sediments

were deposited. This uplift was confirmed by the presence of fault terraces in

the marine sandstone layers thus allowing for the erosion of the sandstone

layers and the deposition of conglomerates to occur. The normal fault noted

from the unit has a strike and dip of N45oW and 65oNE respectively.

Suso Outcrop

General Description

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SW

Figure 12. The Suso outcrop of Brgy. Nalvo, Sta. Maria, Ilocos Sur, characterized by visible columnar jointing. Note the different orientation of the joints emphasized by the red lines.

The road leading to the coastal barangay of Nalvo, Santa Maria, when

passing from Manila North Road, reveals the Suso Outcrop near the village.

This hazel-colored outcrop, located at 17°21'48.8"N 120°27'18"E, of around

15 meters high is primarily showing columnar jointing, indicating a subaerial

lava flow. These cooling joints are irregular in orientation, and secondary

deformation is also visible. The columnar joints follow a normal cooling

sequence because of the stratigraphic positioning of the cooling structures,

i.e. the colonnade jointing shows the bottom of the flow, while the

entablature jointing is seen at the top.

Lithology and Structures

Upon closer inspection, the cooling joints are uniformly composed of

porphyritic andesite, with some of its crystals forming a pattern parallel to

each other. Some of rocks have secondary calcite veins present, as

evidenced by the weak effervescence of the rocks. There are also localized

pyrite grains, as an accessory mineral of igneous rocks.

Petrographic analysis of a thin section was performed on a sample from the

outcrop. The analysis shows two major minerals: amphibole as phenocrysts

and plagioclase feldspar as the main groundmass mineral. The amphibole

crystals follow a preferred orientation, known as “trachytic” texture.

Secondary calcite veins are evident in the thin section, as well as some

opaque accessory minerals.

Evident columnar joints are found to have different attitutes from the left to

the right of the outcrop (shown in Fig. 12). The leftmost portion is

characterized by joints generally dipping to the northeast while the right by

southeast dipping joints. Strike and dip measurements of joints at leftmost

and rightmost portions are N 60° W, 55 NE and N 15 W, 62 SW, respectively.

Interpretation

Based from the consistent lithology from all sides of the unit, the Suso

Outcrop is formed by at least one episode of lava flow exposed to subaerial

conditions allowing the flow to rapidly cool. The parallel pattern of crystal

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grains seen in the andesite is the characteristic of a preferred orientation

(trachytic texture). This type of texture is indicative of flow direction; parallel

to the direction of the lava flows, and conversely perpendicular to the cooling

joints. As the consequence of cooling, the thermal contraction of the

solidified lava flow has formed gaps that are cross-sectionally polygonal,

usually hexagonal in shape, between the rocks. The difference in cooling

rates is crucial in determining the structure of the cooling joints: the rocks

near the surface are cooled quicker, forming entablature jointing, while the

later cooled rocks not directly exposed develop the colonnade structure. An

important clue in the history of this outcrop is the irregular orientation of the

cooling joints. It can be inferred that the paleotopography during the

extrusion of the lava flow was not flat or horizontal.

Although the volcanics along the road to Kapurpurawan and the Suso

Outcrop are both mainly andesitic lava flows, it is important to note and

compare their natures of deposition: 1) the Kapurpurawan lava flow has at

least more than two cooling events in contrast to at least one in Suso

outcrop, 2) the former is extruded sub-aqueously then sub-aerially, while the

latter is mainly deposited in subaerial environment, and finally 3)

Kapurpurawan’s paleotopography was nearly horizontal as inferred from the

uniform trend of the joints, while Brgy. Nalvo’s wasn’t.

Santiago Outcrop

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SE

Figure 13. The side of the outcrop facing the road (Left) Red lines delineating traces of bedding planes (Right) Red lines outlining the olistolith. Height of scale: 1.74meters

General Description and Stratigraphy

The outcrop is located along Bucong Bridge, facing southwest with a height

of 15 meters and length of around 30 meters (Fig. 13). The outcrop is divided

into two parts, one at the front (facing the road) and another one at the back.

From afar, the front side of the outcrop seems to have two units namely the

folded beds of alternating brown and gray layers at the bottom and a

conglomerate unit with megaclasts overlying it. Upon closer inspection, the

folded beds consisting of interbedded shale, siltstone and sandstone show

irregular folding (see Fig. 13 Left) indicative of soft-sediment deformation

rather than tectonically induced folding. Erosional features found in these

beds include spheroidal weathering and desiccation cracks. Moreover, it was

observed that this unit is truncated at both sides of the outcrop facing the

road.

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Figure 14

NW

NW

Looking at the back portion of the outcrop, a massive light brown sandstone

deposit with clasts of what seem to be large boulders can be observed. It is

around the same size as the front, around 15 meters tall and 30 meters wide.

Near the base of the outcrop, it was concluded that the sand was very fine-

to fine-grained. It contains different sizes of rocks, from gravel to boulder-

sized clasts. Most clasts are made up of mudstone, while others were made

up of beds of sandstone, with some evidently rotated. Limestone clasts which

consist of planktonic foraminifera, echinoid spines and coral fragments were

found in this unit. Furthermore, a sedimentary dike (see Fig. 14) was

observed in the outcrop formed from the sand-filling of a fracture in the rock.

Interpretation

The entire outcrop is a slide deposit formed by mass flow processes

specifically submarine slides where coarser debris move as avalanches.

Debris include large blocks of rocks up to tens of meters in size called

olistoliths (Nichols, 2009). This claim is supported by the discontinuity and

soft-sediment deformation of the interbeds suggesting that these water-

saturated rocks are rapidly deformed during movement downslope. It is also

evidenced by the rotated olistoliths of the interbeds found at the back

portion of the outcrop.

This claim places the outcrop on a slope apron, depositional systems found

on continental slopes characterized by mass flow processes (Nichols, 2009).

The dominance of planktonic forams in the limestone clasts further supports

this claim. Together with the interbeds and other olistoliths, a trigger such as

an earthquake separated these rocks from the shelf margin and was

transported down to the base of the continental slope. In conclusion, the

Santiago Outcrop is a chaotic mass of olistoliths known as an olistostrome.

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Tagudin-Cervantes Outcrop

Figure 15. The outcrop at Tagudin-Cervantes road showing an unconformity (green line) and a fault

(red line).

General Description

A large outcrop facing southeast around 40 meters tall from the road is

visible at the Tagudin-Cervantes Road along the Suyo River on Suyo town,

Ilocos Sur. This outcrop is divided into two defining units: an underlying light-

colored interbedded unit of sandstones and mudstones, and an overlying

dark massive conglomerate unit. The presence of talus is evidence for

22

NE

weathering, but primary structures are still visible, such as scour marks,

climbing ripples and convolute laminations. Moreover, measurement of the

attitude of the interbedded unit reveal a strike of N 27° E, and dip of 21° NW.

Stratigraphy and Structures

Closer inspection reveals that the interbedded unit of the outcrop include

alternating non-calcareous layers of A) normal graded coarse to very coarse

sandstone bed with defined scour marks at its base, B) laminated medium to

coarse sandstones, C) cross-bedded sandstone D) planar mudstone

beds/laminae, and sporadically E) massive mudstone. Most, if not all, layers

do not follow one another. As shown in Figure 16A, layer C overlies unit A and

underlies layer E.

23

Figure 16.sequence.laminations

The five-fold sequence mentioned earlier is characteristic of turbidite

deposits, called a bouma sequence. Incomplete bouma sequences in this

outcrop overlie each unit with increasing thickness per sequence, until they

were abruptly truncated by the darker-colored rock identified from afar as

paraconglomerate. From the sharp change in lithology and grain/clast size,

the contact between the two units is a disconformity.

Noteable primary structures include convolute laminations, climbing ripples

and scour marks. As shown in Figure 16 D, convolute laminations are soft-

sedimentary structures indicative of subaqueous slumping and dewatering of

finer grained sediments as coarser grains overlie these deposits. Climbing

ripples (Fig. 16 C) indicate high rates of deposition resulting to the

preservation of both the stoss and lee sides of the ripple.

Secondary structures present in the outcrop include two normal faults (Fig.

16B) of opposite dip directions cutting through both the interbeds and

conglomerate, forming a graben, as well as numerous fractures and

conjugate joints caused by brittle deformation. The two faults have attitudes

of: N 59° W strike, 69° NE (southwest flank of the outcrop) and N 45° W

strike, 46° SW dip located at the center.

Interpretation

The interbedded unit was once part of marine environment, specifically at

the base of the continental slope where sediments are carried down by mass

transport processes, called turbidity currents. From the top of the continental

shelf, passing through submarine canyons, sediments are carried by these

currents before being deposited in a submarine fan.

Because the sediment mass is under suspension, it allows for larger and

heavier grains, like granules and very coarse sand, to settle down first

forming normal graded beds (Layer A as discussed previously). Layers B-E

are later deposited by progressively waning currents. This deposit is episodic,

because it is repeated throughout the outcrop for a number of times. Related

24

to this rhythmic deposition is 1) the presence of scour marks on every

massive sandstone bed (the first in the Bouma sequence), and 2) the

intermittent absence of some of the Bouma units on the turbidites, both of

which imply the erosion of the previous turbidite deposit. What is different

though for every turbidite sequence is that there is a trend of increasing bed

thickness going upward, suggesting an increasing amount of sediment

supply. This increasing sediment supply is indicative of a landward migration

of the shoreline.

Due to the active tectonic setting of the Philippines, uplift might have

exposed the turbidite deposits as shown by the sharp shift of lithology into

conglomerate. Presence of evident faults and fractures supports this claim.

25

Major Outcrops

Solvec Point

General Description

Situated in the Narvacan Quadrangle of Ilocos Sur, the Solvec or Sulvec Point,

is a huge outcrop from the exposed side of the Heroes Hills (shown in Fig. 17

Left) down to the outcrop’s extension towards the coast. To investigate the

extents of the outcrop, eighteen groups were dispersed and assigned to

different stations from the drop off point (see Fig. 17 Right). From the

consolidated observations, four main lithologies were identified namely,

clastics of interbedded mudstones and sandstones, encrusting corals, diorite

and andesite intrusion.

Section Description

26

Figure 17. (Left) A photo showing the extents of the outcrop from the highly weathered portion of the hill down the road near the coast and the assigned section (yellow pin) (Right) Section of the Narvacan topographic map; drop off point (yellow circle)

The station assigned to the group is located near the shore, 17° 27’ 10.5’’ N,

120° 25’ 52.9’’ (see Fig. Left). The section is a 1.91 meter thick gray unit of

beds with a strike and dip of N 22 E and 47 SE. It is composed of two fining

upward sequences of dominantly massive beds of sandstones with grain

sizes ranging from very fine to coarse sand composed of lithic fragments,

quartz and feldspars. Within those two sequences are rip-up rounded clasts

of maroon mudstones (Fig. 18 B) and clasts of intercalcalating layers of thinly

bedded mudstone and sandstone (Fig. 18 C). A set of fault set was observed

in the outcrop with one fault oriented N 37 E and 66 SE, which suggests a

tectonic uplift exposing these rocks.

Geologic History

The assigned section underlies a unit composed of intercalating layers of

thickly laminated to thinly bedded dark maroon mudstone and thinly to

medium bedded normal graded light gray sandstone observed by Group 1

(Sisracon, et. al). Conversely, it overlies a unit consisting of interbedded

sandstone and mudstone and massive sandstones intruded by an andesitic

sill intrusion as seen by Group 8 (Virrey, et. al).

The section assigned to the group belongs to the clastics of interbedded

mudstones and sandstones, one of the major lithologic units identified by the

class. These thick layers of interbeds suggest an abundant supply of

27

Figure 18. (A) The outcrop showing the prominent tilt of the beds, (B) rounded mudstone clasts, (C) clasts of intercalcalating mudstone and sandstone

A C

B

sediments that are rhythmically deposited at the basin of deposition.

Moreover, it is believed to be deposited by turbidity currents as evidenced by

normal graded sandstones with erosive bases and overlying laminated

sandstone and mudstone units (bouma sequence units).

Multiple exposures of this lithologic unit were observed to be intruded by a

dioritic magma (as seen by Group 13, Cacho et. al) characterized by the

baking and chilling margin on the contact between the two units. Located on

the other side of the road is a huge outcrop of highly jointed diorite rocks

similar to the observed intrusions. This suggests a rather large intrusive body

like a batholith underneath which can produce these units. Based from the

Principle of Cross Cutting Relationships, the diorite rocks are younger than

the clastics it had intruded.

Several groups such as Group 5 (Muyco, et. al) have observed andesitic

apophysis of porphyritic andesite intruding the diorite and andesitic sill

intruding the clastics (by Group 15, Florendo et.al). The latter was identified

as a sill because of its concordance with the attitude of the beds and was

distinguished from a flow because of the baking and chilling margins on both

sides of the intrusion. Using the same principle, the andesitic intrusion is

younger than the diorite and clastics it cuts. The two intrusions may have

originated from a single source and the difference in lithology may be

explained by differentiation of the magma after the emplacement of the

diorite.

Uplifted corals, mainly composed of the scleractinian taxa, in their upright

position encrust all the previously discussed units. At least three groups have

seen an angular unconformity between the clastics and this unit. Moreover,

diorite clasts within the limestones were observed suggesting an erosive

28

Youngest

Oldest

contact. By the Principle of Inclusions, the clasts are older than the

surrounding rock.

As shown in the figure above, a generalized cross-section was constructed to

illustrate the huge outcrop. At first, the interbedded sandstone and mudstone

unit was deposited by turbidity currents and was intruded by a dioritic

magma. These units were intruded by an andesite sill followed by an uplift

evidenced by the tilting of the beds and fault sets found throughout the

outcrop. All together these units were overlain by limestone formed by the

encrustment of coral reefs. The upright position of the corals indicates the

top of the unit. By the Law of Superposition, the encrusting coral unit is the

youngest. A stratigraphic column (shown in Fig. 19) was created to represent

the relative ages of the units of the outcrop.

San Juan Outcrop

29

Encrusting coral reefs

Interbedded clastics

Diorite

Andesite

Encrusting coral reefs

Andesite

Diorite

Interbedded clastics

Youngest

Oldest

Figure 19. Generalized cross section of the outcrop and a proposed stratigraphic column

General Description

Just after the boundary between Bacnotan and San Juan, La Union, loosely

located on MacArthur Highway, San Juan, La Union with coordinates 16°41’

N, 120°20’ E, lies a rock formation with a tilted sequence. This part of the

large outcrop is measured to be around 15 meters in height and 8 meters in

length. From afar, the segment is made up of various interchanging beds

that dip to the southeast which is truncated by a scoured base filled with

clasts.

In this part of the fieldwork, the members of the group are expected to

record their data on a stratigraphic log of at least 1.5 meters. A total of 1.9

meters was carefully analyzed by the group.These observations are recorded

on the stratigraphic log attached to the report (see Appendix).

30

SE

Figure 20. (A) The outcrop with the red line delineating the bedding planes and the red curve tracing a disconformity (B) Sample showing mottling (C) Branching trace fossil. Scale: 1.5 meters

A

B

C

Section Description

The section of the outcrop assigned to the group is composed of interbeds of

mudstone and fine to medium-grained sandstone overlain by a red, organic

rich layer of sanstone. The mudstone beds are thinly bedded at first, but

increase in thickness from bottom to the top. All mudstone beds exhibit

spheroidal weathering, and most contain fossils, like foraminifera, and

organic matter. Most of the mudstone beds contain horizontal burrows. A few

beds contain vertical burrows, while one part contains a branching burrow

(Fig 20 C).

The sandstone beds found here are thickly bedded at first, then alternates

between medium to thinly bedded. Most of these beds are indurated,

decreasing in hardness as the layers progress upward. Some sandstone beds

contain foraminifera and/or organic matter. The beds also exhibit convolute

laminations, as shown in the figure above.

Different groups have also observed the different parts of the large outcrop.

Most have the same findings as this group has: interbeds of sandstone and

mudstone, with frequent fossils and trace fossils. From the discussions, a

shift from sandstone dominated sections to mudstone dominated units was

inferred. Overlying the interbeds are polymictic conglomerates, consisting of

igneous clasts such as diorite.

Interpretation

31

Figure 21.

The description of the outcrop is characteristic of a deep marine depositional

environment with associated turbiditic sequences. Turbiditic sequences are

deposited near submarine canyons and is caused by episodes of mass

transport events called turbidity currents, wherein a suspension of sand and

mud-sized sediments are carried down to the base of the continental shelf

and deposited there, with the heavier sand grains settling down first. Not all

of the bouma units are seen, probably due to the short intervals of the

turbidity currents which erode the previous deposits, and the distance of the

turbidity current deposit from the submarine fan. The horizontal burrows

found in most of the mudstone layers are further evidence of calm water

conditions related to deep marine environments. The conglomerate with a

visible erosive base marks a disconformity in deposition. This is probably due

to a landward migration of the coast caused by uplift. Additional tectonic

stresses may be also responsible for the tilting that is seen at present in this

area.

According to Dimalanta and Yumul (2009), the San Juan outcrop is part of the

Amlang Formation, which Aurelio and Peña (2008) reports as “turbiditic

sandstone and shale, with minor conglomerate” and tracks it to Bacnotan

town, which borders San Juan town to the north.

Northern Cement Corporation Quarry

32

Figure 22. The NCC quarries as pictured in Google Earth. Sapid Creek is seen west of the shale quarry.

General Description

The currently operational cement quarry of Northern Cement Corporation in

the southwestern foothills of Cordillera Mountain Range at Sison, Pangasinan

has a very large perimeter of two main rocks mined for the production of

cement, which are wholly limestone and a unit of interbedded limestone,

shale and minor conglomerate. These two rocks are separated by elevation:

the limestone deposit can be seen on the hills extending to the base (as

shown in Fig. 22), while the interbedded unit, described mainly as a shale

quarry (see Fig. 22) by the NCC, is found below the limestone, down onto a

nearby creek that traces the boundary of the quarry.

According to measurements from Google Earth (imagery date: 8/14/2014),

the limestone quarry has a top elevation of 475 meters and base of 190

meters, giving a vertical thickness of 285 meters. This cream-colored to gray

limestone formation is massive to very thickly-bedded and dips to the

southwest. The beds seem to have no thickening trend.

33

N

Figure 23. (Left) Limestone quarry with delineated bedding planes. Backhoe encircled to scale. (Right) Pelecypod fossil

When inspected, the limestone revealed many macrofossils and other

bioclasts, such as pelecypods, as seen in Figure 23, gastropods and other

molluscs, corals, etc. No sedimentary structures can be seen, aside from the

very thick planar bedding to massive structure. Because of the steepness

and nature of the quarry, the fieldwork was confined to a terrace-like area at

the base of the quarry.

Figure 24. (Left) Shale quarry, with evident bedding planes (Right) A portion of the section assigned

to the group

The brown colored-shale quarry is a sequence of alternating beds of

crystalline limestone, calcarenite, conglomerate, shale and massive

mudstone. It is approximately 15 meters in height. It is of note that the

outcrop has a northeast dip direction, which is opposite of the southwest dip

34

SE

SE

of the limestone. This can be explained by the folding and faulting present on

the shale outcrop, suggesting tectonic deformation.

35

Section description

The assigned section for the group is located in the right part of the

NCC Quarry outcrop, just behind where the batch took pictures after the

fieldwork. It is facing SW and contains beds striking N32°W and dipping

36°NE. The assigned section is composed of alternating beds of calcarenite

and conglomerate. The conglomerate is thick with pebble to cobble sized,

subrounded clasts of porphyritic andesite, diorite, lithic fragments, and

bioclasts set in a sandy and calcareous matrix. Gastropod shells, forams,

coral fragments, horizontal and vertical burrows were found in some parts of

the section. The contacts between beds are sharp and distinct.

Starting from the bottom to 2 cm, the bed is identified to be calcarenite

with medium-grained sand size which is brown in color and observed to

contain forams. The next bed is measured to be 4 cm thick and identified to

be calcarenite with fine-grained sand size which is light gray in color. The

next one is 4 cm thick and identified to be a calcarenite with coarse grained

sand size found to be brown in color and friable. Then a friable calcarenite 17

cm thick containing benthic forams and gastropod shell was deposited

followed by a 16 cm thick polymictic orthoconglomerate with clasts of

porphyritic andesite, lithic fragments, coral fragments, shell fragments,

echinoid spines and wood fragments set in fine to medium calcareous sand.

Reddish polymictic paraconglomerate with 21 cm was deposited on top of it

set in fine grained calcareous sand. Another 21 cm that is friable light gray

calcarenite which contains benthic forams, gastropod shells and convolute

lamination observed follows. An 18 cm thick polymictic paraconglomerate

with same clasts found in below is then identified. The second to the last bed

is a 6 cm calcarenite bed found to have horizontal burrows and wood

fragments. The last layer observed was a crystalline limestone which was

measured to be 8 cm thick. The total thickness of beds studied for the

detailed stratigraphic log is 1.13m.

36

Interpretation

Based on the data, a possible interpretation of the area is that it was once

under a shallow marine environment dominated by calcite-bearing

organisms, such as corals, shells, etc. This led to deposition of the limestone

currently being mined. The macrofossils give credence to the shallow marine

interpretation. This limestone dominance was later replaced by an

environment with active sedimentation processes, such as erosion. Calcite-

bearing organisms don’t thrive in areas with high sediment supply.

From here, sediments are carried down by sediment gravity transport

processes. The clasts included in conglomerate include terrigenous rocks,

such as porphyritic andesite, which means that fluvial processes may have

contributed to the kind of sediments being deposited. During breaks between

clastic depositions, calcite-bearing organisms were allowed to flourish again,

and causing the deposition of limestone. These processes: sediment gravity

transport, bioclastic deposition, and calcite precipitation, are repeated many

times in the sequence of the outcrop.

The last phase in the formation of this outcrop is uplift and other tectonic

processes, due to local tectonics. Uplift exposes these rocks above sea level,

while faulting and folding are caused by ductile to brittle deformation of the

rocks related to tectonics. The presence of tectonic deformation is evidenced

by the different dip directions seen in the two outcrops, which may suggest

folding or faulting between them, as well as the tilting and microfaults seen

in the shale quarry.

37

REFERENCES

Esguerra, N., et. al.(2008). Characterizing the Environmental Effects of the Quarrying Industry: The Case of Strategic Quarry Sites in the Ilocos Region. UNP Research Journal , 38-50.

Fossen, H. (2010). Structural Geology. Cambridge: Cambridge University Press.

Ilocos Norte. (n.d.). Retrieved May 16, 2015, from http://tourism-philippines.com/ilocos-norte/

JSP (2008). Ilocos Norte Sand Dunes National Geological Monument. (handout given during the fieldwork)

NAMRIA - Topographic Maps.(n.d.). Retrieved May 16, 2015, from http://www.namria.gov.ph/topo50Index.aspx

Nichols, G. (2009). Sedimentology and Stratigraphy (2nd Ed). Oxford: Blackwell Science.

Peña, R. (2008). Lexicon of Philippine Stratigraphy, 2008.Mandaluyong City, Philippines: Geological Society of the Philippines.

38

APPENDIX

Appendix A. Detailed stratigraphic log of assigned section in San Juan

39

40

Appendix B. Detailed stratigraphic log of assigned section in NCC Quarry

41

Appendix C. Other rock samples

42

Figure 25. Luna Beach gravel deposits. (Left) Sandstone, diorite, andesite clasts. (Right) Coral

fragment.

Figure 26. Kapurpurawan rock samples. (Left) Porphyritic vesicular basalt. (Right) Porphyritic vesicular

andesite.

Figure 27. San Nicolas rock samples. (Left) Leaf fossil. (Right) Gastropod fossil.

43

Appendix C. Other rock samples (continued)

Figure 28. Suso outcrop. (Left) Porphyritic andesite with trachytic texture. (Right) Thin section. Photo

by Ynna Aquino.

Figure 29. Tagudin-Cervantes outcrop. (Left) Planar laminated sandstone. (Right) Massive very coarse

sandstone.

44