6. SITE 187
The Shipboard Scientific Party1

SITE DATA
Date Occupied: 12-13 Aug71.
Position:
51°06.6'N
173°57.2'W
Water Depth: 4577 meters.
Penetration: 370 meters.
Number of Holes: One.
Number of Cores: Four.
Total Core Recovered: 6.8 meters.
Acoustic Basement:
Depth: 160 meters
Nature: Deformed sediments
Velocity: Approximately same as Site 186, 2.0 km/sec.
Age of Oldest Sediment: Early late Miocene.
Basement: Miocene deformed sediments.
SUMMARY

The sediment sequence at Site 187, located 2.3 miles
southeast of Site 186 (Figure 1), consists of diatomaceous
silty clay that is older but otherwise virtually identical to that recovered at Site 186. Only four cores
were attempted to a total depth of 370 meters; they
contained sediment of Pleistocene, lower Pliocene, and late
Miocene age. Limestone fragments (one of which contains
fecal pellets), silty mud, and an ice-rafted (?), rounded
graywacke pebble were recovered from the core catcher ofCore 1 at 164 meters. The two limestone fragments containa lower Pleistocene flora and probably caved from higher inthe hole, as the associated silt is of early Pliocene age.
Upper Miocene disturbed (probably by coring operations)and indurated silty clay (claystone) with associated diatomaceous sediment was recovered between 175 and 370
meters.
Site 187 is located above the "acoustic basement"
underlying the outer ridge of Atka Basin, Aleutian Terrace.
This ridge is also the summit of the steeply sloping innerwall of the Aleutian Trench. Drilling at this site shows that
1David W. Scholl, U.S. Geological Survey, Menlo Park, California; Joe S. Creager, University of Washington, Seattle, Washington; Robert E. Boyce, Scripps Institution of Oceanography, LaJolla, California; Ronald J. Echols, University of Washington,Seattle, Washington; Timothy J. Fullam, Standard Oil Company ofCalifornia, La Habra, California; John A. Grow, Massachusetts
Institute of Technology, Cambridge, Massachusetts; Itaru Koizumi,Osaka University, Osaka, Japan; Homa J. Lee, Naval Civil Engineering Laboratory, Port Hueneme, California; Hsin Yi Ling, Universityof Washington, Seattle, Washington; Richard J. Stewart, Universityof Washington, Seattle, Washington; Peter R. Supko, Scripps
Institution of Oceanography, La Jolla, California; Thomas R.
Worsley, University of Washington, Seattle, Washington.

at least part of the acoustic basement is deformed sedimentary rock as old as late Miocene. These deposits
presumably accumulated in Atka Basin. Near Site 186,
beneath this basin, stratigraphically equivalent beds lie
deeper than 926 meters. The structural displacement of late
Miocene beds between Sites 186 and 187 must be at least
750 meters. The source area for the allochthonous block of
middle Miocene silty clay cored at Site 186 in beds of early
Pliocene age is presumably the ridge drilled at Site 187 (seeGrow, this volume).
BACKGROUND AND OBJECTIVES
Description

Site 187 is located over the inner part of the outer"bedrock" ridge of Atka Basin, Aleutian Terrace (4577 m,corrected). The site is just 2.3 miles southeast (132°T) of
previous Site 186. The reference profile (Figure 2) revealed
about 100 to 150 meters of pelagic debris overlying an
acoustically unresolvable "basement," which could either
be deformed sedimentary debris or possibly vesicularvolcanic rocks. Gravity data imply that the outer ridge is
not underlain by exceptionally dense material, hence
deformed sedimentary deposits were expected.

50c
176°

172°

Figure 1. Base map showing the location of Site 187.

279

SITE 187

10 km

2-waytime,sec.

Near 186-6" ^4'vH
0;i;
.Wz

51

i.7

Figure 2. Reference seismic reflection profile, obtained by E. L. Hamilton, 2 Aug 70.
Objectives

Drilling Program

A considerable body of circumstantial evidence existsthat implies that the outer ridge, which is also the upper
part of the inner wall of the adjacent Aleutian Trench, may
be underlain by oceanic offscrapings. The principal objective of Site 187 was to (1) determine the nature of therocks underlying the ridge, (2) establish their relation to thesedimentary section drilled at Site 186, and (3) assess thepossibility that oceanic offscrapings may be present.

Site 187 was occupied from 1215 hrs 12 Aug 71 (beaconaway) until 1200 hrs 13 Aug 71 with alternate coring andwashing from the sea floor to a subbottom depth of 370
meters. At this depth a diatomaceous clayey silt, of earlylate Miocene age, lithologically similar to that encounteredat Site 186 was cored. Coring speed, which was slow, andthe necessity to disembark personnel at Adak on 14 Aug 71
required that the hole be abandoned.
Using Matthews Tables and information from the U.S.
Navy Undersea Research and Development Center, thesonic depth of 2342 fms was corrected to 4538 meters
giving a water depth of 4544 meters and a drill-floor depthof 4554 meters. Considering the past errors in thiscalculation, the bottom below drill floor was estimated at4587 meters. This estimate then, gives an estimated depth
below sea level of 4577 meters. This is the accepted depth,
inasmuch as, in order to save time, the bottom was notcored. No difficulties were encountered in drilling this hole.
A coring summary is given in Table 1.

OPERATIONS
Pre- and Post-drilling Survey

Site 187 is located 2.3 miles southeast (132°T) of Site
186 along the track of the reference profile collected by E.
L. Hamilton on 2 Aug 70. The ship was moved to this sitewith the drill string suspended below the ship, therefore noseismic equipment was streamed. A new beacon wasdropped at 1210 hrs on 12 Aug 71. The finally accepted
position is: 51°06.62'N; 173°57.23'W. The reference
seismic profile is shown in Figure 2. No air-gun profile wasmade coming onto the site, but a post-drilling survey was
made which passed through Sites 186 and 187 (Figure 3). A
map showing the surveys and track and site locations isshown in Figure 4.

280

LITHOSTRATIGRAPHY

The deposits recovered from Site 187 are dark brownish
gray and olive gray diatomaceous silty clay, identical tothose recovered at Site 186. An ice-rafted(?), roundedSITE 187oLO

COCO COalQ
LUi i i

2465
1600

HDG

COQ.
LU
GO i—Q.

I—ILuQFigure 3. Reflection profile obtained by Glomar Challenger upon departure from Site 187.
pebble of graywacke was recovered from the core catcher in
the Interval 164 to 173 meters along with mud of early
Pliocene age and two angular pieces of lower Pleistocene
limestone (one of which included fecal pellets) probably
derived from higher levels. In Core 2 a light gray vitric ash
was recovered (174 m) in beds of late Miocene age. The
final core recovered from 361 to 370 meters contains
highly disturbed layers of brittle claystone in an otherwise
soft section of diatomaceous silty clay.
PHYSICAL PROPERTIES

A limited number of GRAPE measurements and shorelaboratory density determinations were made on the"sediment from Site 187. These are presented on the site
summary sheet. No trends are apparent over the range
tested.

PALEONTOLOGY
The four cores taken at Site 187 consist of diatomaceous
silty clay; the core catcher of Core 1 contained twolimestone pieces, one of which included fecal pellets. Onlysiliceous fossils have been found. The limestone fragment ofCore 1 is Pleistocene in age and is thus not indigenous tothe silty clay, early Pliocene, within which it is enclosed.
This fragment presumably fell down the hole from a higher
stratigraphic level. Silty clay of the remaining cores are oflate Miocene age.

281

SITE 187
174°10'W

51O15'N

Figure 4. Track of the Glomar Challenger in the vicinity of Sites 186 and 187.
Calcareous Nannoflora

Although some of the cores at this site contain limestone
bands, the site is apparently barren of nannofossils.
Radiolaria and Silicoflagellates
Occurrences of radiolarians and silicoflagellates fromSite 187 are shown in Table 6, Chapters 27 and 28. InCores 1 and 3 limestone and clay contain different
microfossil assemblages and, in the case of Core 1, theassemblages are even of different ages. A few radiolarians
were found from other samples at this site, but without any
index taxa.
As for silicoflagellates, the limestone of Core 1 yields
varieties of Distephanus speculum and Dictyocha subarctios, apparently indicating Pleistocene age.
Diatoms
The limestone of Core 1 contains many diatoms ofPleistocene age. Diatom assemblages of associated silty clay
indicate an early Pliocene age. The flora of Core 4 isdominated by Coscinodiscus marginatus forma and Denticula kamtschatica, and it seems to be older than Core 28
of Site 186.

282

CORRELATION BETWEEN REFLECTION PROFILEAND STRATIGRAPHIC COLUMN

See Correlation Between Reflection Profile and Stratigraphic Column report for Site 186.

TABLE 1
Coring Summary - Site 187
RecoveredCore

Cored Interval
Below Bottom(m)Cored(m)Wash
1164-1739173-182

2
Wash
3Wash
4(m)

(%)

0.0

93 pebbles
in catcher
3.0

33.3

267-276

91.5

16.7

361-370

92.3

25.6

366.8

18.9SITE 187

DENSITY VELOCITY
1o o •~(_> Oi

3100••

WITH
VOLC.
SANDS

9σ
CSI-1'

DIATOM
OOZE

8• o1— CSI

SILTY
WJJH_ASH_45

CU<LU00—1LU•

1EISTOCE!

LU
Q

•X

•.a '

1 1

* *

12
SEMIINDURATED

13σ
•σ CO

1415
164000—><C•—«••» —^

VOLC. "SANDSS WITH
L! CALCITEoCEMENT1720•1 i 1z
oo1a.

••-

Q

LO

WITH
VOLC.
SAND

600

221.0—•2324o• o -•ro. o . 21;o.5—-• •••••• •••-M. MIO.
EXOTIC
BLOCK
RARE
ASH

M.MIC'LU
Z
LU
O

25o_J
Q_os• 26

V.E. = 1227o• o

NORTH [TERRACE)

SOUTH (TRENCH)σ>ft

VÖLC~!
SAND(BURROWED)-V

28
HOLE TERMINATED

i,/Figure 5. Correlation of the seismic reflection profile with physical properties and the lithologic column, Sites 186 and 187.

283

NJ
OO
DENSITY(gm/cc)Mean GRAPE Density, +Shore Laboratory Density, x
Water Displacement Density,DEPTH
(in)
-150-

LITHOLOGIC DESCRIPTION

limestone exotic,
graywacke pebbleDIATOMACEOUS
SILTY CLAY,
dark gray ando1ive grayZ.02.0COMPRESSIONALWAVE
VELOCITY(km/sec)
52.0

SITE 187
Core 1LITHOLOGYCoreCatchermM
M

LITHO.SAMPLE

PRES.
G

Cored Interval:164-173

METERSABUND.

D A
PF
BF
N
R R
S R

FOSSILZONECD) Denticula seminaeCS)- D. kamtschatica

1 PLEISTOCENE - LOWER PLIOCENE

AGE

FOSSILCHARACTER

DEFORMATION |HoleSECTION

Site 187

LITHOLOGIC DESCRIPTION

core catcher sample only:
1) angular pieces of LIMESTONE (5-10 cm)
dusky yellow (5Y 6/4) and med. light gray (N6)2) rounded GRAYWACKE pebble (5-10 cm)
dark gray (N4)3) small amount of diatomaceous silty clay

Explanatory notes in Chapter 1Core 22Core
CatcherExplanatory notes in Chapter 1286

LITHO.SAMPLE |DEFORMATION |PRES.

ABUND.

FOSSILZONECD) Denticula kamtschatica

1

LITHOLOGY

1 1 1 1 1 1 1 1 1 1 1 1 1 1

AGE

Cored Interval:173-182

1 1 l l | 1 I 1 I | i I I 1

UPPER MIOCENEFOSSILCHARACTER

METERS

HoleSECTION

Site 187

LITHOLOGIC DESCRIPTION-105
VOID147

VITRIC ASH, light bluish grayCore Catcher: Basic lithology:
DIATOM BEARING SILTY CLAY
DA Gpp _dark greenish gray (5GY 4/1)
BF 112 N - R R M with thin (1-2 cm) layers of SAND,
S R M
greenish black (5G 2/1)
and diatom rich zones, olive gray (5Y 4/1)

SITE 187

WET-BULK DENSITY, gm/cc
123SECTION
CM-0

11 i i i i i i i ' i i ' ' i « ' " i

POROSITY,%
1000GrainDensity
2o/rr
1-h-25.

2--50
3-4-755-1-100-125

8--150187-2-1 187-2-2287

SITE 187
Site 187

HoleCore 3

Cored I n t e r v a l : 2 6 7 - 2 7 6

FOSSILCHARACTER

LITHOLOGY

LITHOLOGIC DESCRIPTIONVOIDDIATOMACEOUS SILTY CLAY
dark gray (5Y 4/1)

0.5—-80
1.0-Core Catcher:
D p Q
PFr i
BF
N _ _
R R M
S R M

CoreCatcher

Slide 1-80
30% diatoms
25% silt
5% glass
40% clay

XR 1-10074% amorph
6% quartz
7% plag.
6% mica
2% chlor.
4% mont.

black streaks inclined 5-10° to horizontal(D) Denticula kamtschatica
Site 187

Hole

Core 4

Cored Interval: 361-370

LITHOLOGIC DESCRIPTION-140

DIATOMACEOUS SILTY CLAY
olive gray (5Y 3/1 - 3/2)

XR 2-80
77% amorph
5% quartz
7% plag.
6% mica
2% chlor.
2% mont.
TR clinop
TR amphib

Core Catcher: Slide 1-140
D C G 30% diatoms
30% silt
PF _40% clay
BF _N - R R M
S R M
section 2 contains 1-2 cm thick disturbed
layers of black (5Y 2/1) SAND

Explanatory notes in Chapter 1288

SITE 187
WET-BULK DENSITY, gm/cc
12SECTION

3POROSITY,*10001 r r irπ

CMroGrain
Density-a 2 uMO-i

1--25

2--50
3-4-755-

6--100

7-125

8-

9-1

187-3-1 187-4-1 187-4-2

289