Y. Narwiez & S. Petri: Biostratigraphy of the Tal ara Group, Peru .. ., p. 9-13.

Journal of Nannolankton Research, 23, I, 2001.

CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF THE
TALARA GROUP (MIDDLE-LATE EOCENE) IN THE NEGRITOS
AND CARPITAS FIELDS, TALARA BASIN, PERU
Yanina Narvaez* & Setembrino Petrif*Univ. ofSan Marcos, Jr. Gui//ermo Geraldino 1650, Lima 1, Peru;jynrodri@hotmail.com.

fGeoscience Inst., Univ. ofSilo Paulo, PO Box 11348, 05422-970, SP, Brazil

Abstract: A biostratigraphic study of rock samples from the Talara Group (Talara Basin, Eocene of NW
Peru) has allowed the recognition of several calcareous nannofossil ·biozones. Two wells were investigatedfrom the Negritos and Carpitas oilfields, in the NW region of Peru . Biozones identified in the TalaraGroup provide dates of Middle to Late Eocene. Ca lcareous nannoplankton from this stratigraphic unit
have not been previously described. ·Introduction
The Talara Basin is situated in thewestemmost part of Peru in South America.
The Talara Group is a stratigraphic unit withwidespread distribution, and it is the mostrepresentative of the Eocene of NW Peru.
The Talara Group comprises mainly the
Talara Shales, Talara Sandstone and Pozo
Formations. The Talara Shales and Pozo
Formations consist of dark brownish togreenish-grey shales, silts and silty clays.
The Talara Sandstone is composed of
medium- to fine -grained deep-water
sandstones. The Talara Shales contain asection composed of synsedimentary
breccias. This section is interpreted as amarine-slope facies complex that grades into
an inner-shelf transgressive system(Muiioz, 1980).
Biostratigraphic investigations of
this stratigraphic unit, based on benthic.Quaternary81 "O Neogene
Palaeogene

O

Cretaceous·
Paleocene- ~~~n:~nN

ECUADORl
PACIFIC
OCEAN

Figure 1: Geological map of the area, showing locati ons of wells studied (modified
from Palaci os, 1995).

Around 86% o~Jhe smear-slides of both wells yielded
datable nannofloras. Because of the ever-present dangerof downward contamination, due to caving while holes
were being drilled, the highest occurrences (tops) are used
for the recognition of zones.

foraminifers, have been compiled by Gonzales ( 1976) but
calcareous nannoplankton have not been previously
described. The lack of detailed studies of calcareous
nannofossils in the basin seriously limits attempts tocorrelate the biozones. Therefore, the main goal of this work
was to study the calcareous nannoplankton of the TalaraGroup sediments and to provide a biozonation for this unit.

Biostratigraphic remarks
In most of the studied samples of Well 12202, thenannofossils are common and their preservational quality
reveals that the majority of specimens were little affectedby dissolution. Moderate preservation and common torare nannofossil abundances were typical of Well X-2
samples. The distribution of nannofossils, as well as theirabundances and states of preservation, are shown in
Figures 2 and 3. The biozonation used in this study isbased on that compiled by Perch-Nielsen (1985) after
Martini (1971).

Material and methods
Two wells were investigated from the NW region of Peru
(Figure 1). Well12202 (24-621m) is from the Negritos oilfield,with a total number of 63 cuttings samples studied, andWell X-2 (1078-1725m) is from the Carpitas field, from which
36 ditch-cuttings samples were collected. Smear-slides wereprepared using standard methods and were mounted with
Canada balsam. The LM study was undertaken at amagnification of 1280x using a Zeiss Photomicroscope.
Some of the observed specimens are illustrated in Plate 1.
A quantitative analysis was attempted, with atleast 300 specimens counted from the samples ofWelll2202.
However, Well X-2 samples had poor nannofossil assemblages, and so fewer than 300 specimens were counted.

Early Middle Eocene - NP 14
This zone is recognised by the last downhole occurrence(LDO) of Discoaster sublodoensis and first downhole
occurrence (FDO) of Rhabdosphaera injlata. It is recorded

9

Journal of Nannolankton Research, 23, I, 2001.

Y. Narvaez & S. Petri: Biostratigraphy of the Talara Group, Peru .... , p. 9-13 .

Figure 2: Distribution of calcareous nannofossils in Well 12202, Negritos field. Abundance: A = abundant, C = common, F = few, R = rare,
VR = very rare; Preservation: M = moderate, P = poor; Degree of etchingl overgrowth : E/01 = slight, E/02 = moderate, E/03 = strong;
Caving: I = slight, 2 = moderate.

recorded from 276m in Well 12202. The most cmmnon
species in this zone are Sphenolithus radians, Ericsonia
formosa, Discoaster barbadiensis, D. sublodoensis,
Neococcolithes dub ius and Lophodolithus mochloporus.
This zone corresponds to the initial stage of depositionof the Talara Shales Formation. The Talara Groupunconfonnably overlies the Echino stratigraphic unit.

Middle Eocene - NP 15 to NP 16
Zones NP15 and NP16 have been combined due to theabsence of the appropriate distinguishing marker-species(Rhabdolithus gladius, Reticulofenestra umbilica). Thesezones are recognised by the LDO of Chiasmolithus solitus.
The event was recorded at 216m in Well12202. The interval
is characterised by Ericsonia formosa, Coccolithus
pelagicus, Disqoaster barbadiensis and C. solitus. Thesezones comprise part of the Talara Shales Formation. NP16
is also recorded from 1212m in Well X-2. The absence ofmarkers for the base of NP16 in these sections may be
attributable to biogeographic restrictions.

Late Middle Eocene - NP 1 7
The base of NP17 is identified by the FDO of
Chiasmolithus solitus. The base of NP18 could not berecognised because of the absence of Chiasmolithus
oamaruensis. However, Martini (1990) described the last
occurrence of Helicosphaera seminulum towards the topof NP 17. This species was recorded at l38m in Well
12202, and so the top of the studied interval is believed
to lie in NP 17. This interval is characterised by
Discoaster barbadiensis, C. solitus and H. seminulum.
H. seminulum, however, does not occur in Well X-2, andso the base of NP18 could not be recognised here.
Late Middle Eocene to Late Eocene- NP 17 to NP20
These zones are indistinguishable in Well X-2 because of
the abseNce of the appropriate marker-taxa (Perch-Nielsen(1985) commented that Chiasmolithus and Jsthmolithus
are absent or rare in low-latitude sections). The LDO of
Chiasmolithus solitus can be used to determine the baseofNP 17, and Discoaster barbadiensis, the FDO of which

10

Y. Narvtiez & S. Petri: Biostratigraphy of the Talara Group, Peru ... , p. 9-13.

Journal of Nannolankton Research, 23 , I, 2001.

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barbadiensis presentT

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t:-=:::-=.- 1587 8
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r-------- 1617 61644 5=.- --- 1662 4
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Figure 3: Distribution of calcareous nannofossils in Well X-2, Carpitas field. Key as for Figure 2.

marks the base of NP21, is still present at the top of thestudied interval (1122m). The most common species in thisinterval in Well X-2 are Coccolithus pelagicus, Ericsonia
formosa, Reticulofenestra dictyoda and D. barbadiensis.

X-2

NE-SW

Conclusions
Biozones NP14 to NP17 have been identified in Well12202(Negritos oilfield). In Well X-2 (Carpitas oilfield), NP16 toNP20 have been distinguished. Zones NP17 to NP20, andNP 15 to NP 16 cannot be distinguished due to the absenceof marker-taxa. The Middle Eocene succession is generally thicker than the Late Eocene interval (Narvaez, 1998).
The base of NP 17 (Middle Eocene) can be correlated between the two wells. The Talara Group in well 12202 does
not contain nannofossil biozones of Late Eocene age.
The relatively low nannoplankton diversity, andthe absence of certain marker-taxa, from Well X-2, ispossibly the result of greater dissolution at deeper waterdepths than in Well 12202, in which nannoflora aremoderately well-preserved, suggesting that deposition may
have taken place in depths well above the carbonate
compensation depth. However, it is also possible thatcertain marker-taxa are missing as a result ofbiogeographic
distributions.

NPI5TO
NPI6
NPI4

\

\
\

\

\
\
NPI7 TO
NP20

NPI6

Acknowledgements
This research was finahcially supported by the CAPES
Foundation and the Geoscience Institute, Sao Paulo

Figure 4: Correlation between Wells 12202 and X-2.

11

Journal of Nannolankton Research, 23, 1, 2001.

Y. Narwiez & S. Petri: Biostratigraphy of the Talara Group, Peru.... , p. 9-13.

Martini , E . 1990. Tertiary and Quaternary calcareous
nannoplankton biostratigraphy off Peru (ODP Leg 112).
Proc. ODP, Scientific Results, 112: 217-238.
Mufioz, L. 1980. Brechas sedimentarias en la Fm. Lutitas Talara
en el Nor-Oeste Peruano. Boletin de la Sociedad Geol6gica
del Peril, 65: 105-127.
Narvaez, J. Y. 1998. Ocorrencia e distribuifiio de nanofosseis
calcarios no Grupo Talara, Bacia Talara no Noroeste do
Peru . MSc thesis, Universidade de Sao Paulo, Sao Paulo:
114pp.
Palacios, 0. 1995 . Geologia del Peru . Boletin del Instituto
Geologico, Minero y Metalurgico, Lima-Pero, Serie A, 55: 1177.
Perch-Nielsen, K. 1985. Cenozoic calcareous nannofossils. In :H.M. Bolli, J.B. Saunders & K. Perch-Nielsen (Eds). Plankton
Stratigraphy. Cambridge University Press: 427-554.

University (USP), Brazil. We are very grateful to the GMP
SA for the provision of sample material and stratigraphic
information. Thanks are due to Drs P.R. Bown and J.A.
Lees (UCL) and Alfredo Pardo (Senior Geological
Consultant) for critical reading of the manuscript.

References

Gonzales, G. 1976. Bioestratigrafia del Eoceno en la region de
Talara. Engineer's thesis, Universidad Nacional de San
Agustin, Arequipa: 224pp.
Martini, E. 1971. Standard Tertiary and Quaternary calcareous
nannoplankton zonation. Proceedings of the II Planktonic
Conference, Roma, 1970, 2: 739-785.

12

Y. Narwiez & S. Petri: Bi trat igraphy of the Talar Group, Peru ... , p. 9·13.

Journal ofarmolanlaon Rcse rch, 23, I, 2001.

PLATE 1
•

•

•

••

•

Discoaster sublodoensis
426m, Negritos field; l2Jlm
diameter; Early Middle
Eocene; Neg.# 07-40

Helico phaera seminulum
216m, Negritos field ; l3Jlm
length; Middle Eocene;
Neg.#07-15

Discoa ter barbadiensis
240m, Negrito field; l3Jlm
diameter; Middle Eocene,
Neg.# 09-5

Pemma basquense
138m, Negritos field ; lOJlm
diameter; Late Middle
Eocene; Neg.# 04-15

Chiasmolithus solitus
240m, Negrito field;
12.5Jlm length; Middle
Eocene; Neg.# 01 -21

Coccolithus pei/Jgicus
222m, Negrito field; l2Jlm
length; Middle Eocene;
Neg.#04-3

Rluzbdolithus
in.flall:r1.16m,
Negrito field ;l8Jlm length;
Middle Eoceoe;
Neg.#08-27
Zygrhablithus bijugaJus
342m, Negritos field; 10Jlm
length; Early Middle Eocene,
Neg.# 04-19

Micrantholitus vesper
420m, Negrito field ; l5Jlm
diameter; Early Middle
Eocene; Neg.# 08-14

Helicosphaera lophota
228m, Negritos field; l2Jlm
length; Middle Eocene;
Neg.#08-3

Lophodolithus mochloporus
465m, Negritos field ;17.5Jlm length· Early Middle
Eocene; Neg.# 04-35

13..