Tectonic controls on sequence stacking pattern and along-strike architecture in the Pleistocene...

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Sedimentary Geology 18

Tectonic controls on sequence stacking pattern and along-strike

architecture in the Pleistocene Mejillones Formation, northern Chile:

Implications for sequence stratigraphic models

Gino Cantalamessa a, Claudio Di Celma a,*, Luca Ragaini b

a Dipartimento di Scienze della Terra, Universita degli Studi di Camerino, Via Gentile III da Varano, 1-62032 Camerino (MC), Italyb Dipartimento di Scienze della Terra, Universita degli Studi di Pisa, Via Santa Maria, 53-56126 Pisa, Italy

Received 1 March 2005; received in revised form 20 August 2005; accepted 14 September 2005

Abstract

In the Mejillones Formation, a shallow-marine Pleistocene succession of northern Chile, the cyclic stratigraphic record is the

result of the complex interaction of regional uplift, glacio-eustasy, local tectonics, sediment supply, and sedimentary processes.

Stratal geometries, characteristics of sedimentary facies, and nature of sequence-bounding unconformities have been investigated to

evaluate the influence of: (i) intrabasinal, short-term normal faulting on both along-strike variations in sequence architecture and

genetic complexity of key stratal surfaces; and (ii) long-term regional uplift on sequence stacking pattern. The stratigraphic

succession, dissected by small-displacement (few meters) normal faults striking obliquely with respect to the palaeo-shoreline

trends, displays systematic variations in sequence architecture and the nature of bounding surfaces across them. Indeed, depending

on position with respect to the fault plane, two basic types of internal organisation can be recognised in the examined shallow-

marine, almost clastic-starved sequence. Within grabens it consists of a siliciclastic-rich transgressive systems tract (TST), which is

bounded beneath by a transgressively modified, Glossifungites-demarcated sequence boundary (SB/RS), overlain by a mollusc-

bearing falling-stage systems tract (FSST). The erosional downlap surface that separates the TST from the FSST is the regressive

surface of marine erosion (RSME). On the footwall crests the combination of marine regressive erosion, during falls in relative sea-

level, and uplift has resulted in complete removal of the sediments of the TST from these sites, leading to the formation of a

tectonically enhanced basal unconformity composed of the RSME superimposed onto the previous SB/RS (SB/RS/RSME). The

prominent lateral change in component units (systems tracts) and nature of bounding surfaces within the studied sequence is

directly related to the presence of normal faults and indicates that fault activity had a major impact on the sequence stratigraphic

evolution of the Mejillones Formation, enhancing subsidence within the grabens and promoting unconformities in the horsts.

Overall, the Mejillones Formation records a long-term sea-level fall driven by the contemporaneous regional uplift, punctuated

by repeated, high-frequency eustatic sea-level changes. The effect of this superimposition was that glacio-eustatic sequences were

displaced progressively downward and basinward and stacked in a distinct downstepping, tectonically enhanced falling-stage

sequence set, which reflects basin-wide loss in accommodation space. The sequence set is underlain by a composite RSME that

becomes progressively younger basinward and is made up by the lateral and down-dip connection of a series of lower-rank

sequence boundaries including hanging-wall SB/RSs and footwall SB/RS/RSMEs of successive sequences.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Sequence stratigraphy; High-frequency sea-level changes; Tectonics; Pleistocene

0037-0738/$ - s

doi:10.1016/j.se

* Correspondi

E-mail addr

3 (2006) 125–144

ee front matter D 2005 Elsevier B.V. All rights reserved.

dgeo.2005.09.010

ng author. Fax: +39 0737 402 644.

ess: [email protected] (C. Di Celma).

G. Cantalamessa et al. / Sedimentary Geology 183 (2006) 125–144126

1. Introduction

Early sequence stratigraphic models predict that the

internal organisation of depositional sequences is con-

trolled by the complex interplay of changes in accom-

modation at the shoreline (including eustasy and

vertical tectonic movements) and sediment supply

(Schlager, 1993). However, while the way in which

accommodation changes along the depositional dip of

a subsiding basin margin has been fully documented

(e.g. Posamentier et al., 1988), comparatively less at-

tention has been paid to the effects on stratal architec-

ture of local basin factors, such as along-strike changes

in accommodation in response to shoreline-perpendic-

ular faulting (Gawthorpe et al., 1994; McMurray and

Gawthorpe, 2000; Hodgetts et al., 2001; Carr et al.,

2003; Jackson et al., 2005).

In the present study, we illustrate a detailed sedi-

mentologic and a high-resolution sequence-stratigraph-

ic interpretation of Pleistocene sediments from the

north Chilean coastal margin (the Mejillones Forma-

tion) in an attempt to constrain the scales at which local

and regional tectonic mechanisms, sediment supply,

and eustatic sea-level fluctuations may have affected

the internal architecture and the overall regressive

stacking pattern of high-frequency, eustatically driven

depositional sequences. The high-resolution sequence-

stratigraphic analysis of young successions where all

the variables influencing sequence development, such

as basin-physiography, plate tectonic setting, rates and

magnitudes of glacio-eustasy, and sediment supply are

well constrained, provides an excellent opportunity to

evaluate their effective control on stratigraphic archi-

tecture (e.g. Saul et al., 1999; Massari et al., 2002;

Cantalamessa and Di Celma, 2004; Lu and Fulthorpe,

2004; Di Celma et al., 2005) and remains an area of

broad interest in the basin analysis. The outcomes of

this study have direct implications for strike variability

in stratigraphic style along normal faults and on related

sequence stratigraphic analysis. Because the Mejillones

Formation provides a useful model for tectonic controls

on internal architecture and stacking pattern of compo-

nent sequences, the major objectives of this study were:

(i) to determine the processes responsible for the depo-

sitional facies of the Mejillones Formation; (ii) to

define the architecture of the component lithosomes;

(iii) to provide new insights into the relationships

between the sequence architecture, syn-depositional

normal faulting, and eustatic sea-level fluctuations;

(iv) to evaluate how high-frequency eustasy and long-

term regional uplift interact to construct a specific

sequence stacking pattern.

2. Geological, stratigraphic, and palaeoclimatic

setting

The active extensional framework observed along

the western margin of northern Chile (Armijo and

Thiele, 1990; Gonzalez et al., 2003) is inferred to

have been active since Miocene time (Hartley and

Jolley, 1995; Niemeyer et al., 1996). At present, the

most widely accepted mechanism for E–W-directed

forearc extension along this active convergent margin

is subduction erosion (i.e. the scraping off of the con-

tinental crust by subducting oceanic lithosphere), fol-

lowed or accompanied by extensional collapse of the

leading edge of the South American plate toward the

north Chile Trench (Delouis et al., 1998; von Huene et

al., 1999; Hartley et al., 2000). From latest Pliocene–

Early Pleistocene onwards, extension has taken place

along with the active regional uplift of the north Chi-

lean coast (Hartley and Jolley, 1995; Ortlieb et al.,

1996b). Compelling evidence for this uplift is recorded

by: (i) the extensive flight of upper Pliocene and Pleis-

tocene marine terraces preserved along wide coastal

tracts of northern Chile (e.g. Radke, 1987; Ratusny

and Radke, 1988; Cantalamessa et al., 2004); and (ii)

the onland exposure of Miocene to Pleistocene shallow-

marine sediments infilling major hanging-wall basins.

According to Hartley et al. (2000) uplift is driven by the

accretion beneath the forearc, through subcrustal

duplexing, of material removed by subduction erosion.

At a regional scale, average uplift rates appear do not

have exceeded 0.15 m ky� 1 during the whole Quater-

nary (Zazo, 1999), although in some sectors the rate of

vertical deformation has increased to 0.24 m ky�1 since

the Middle Pleistocene (Ortlieb et al., 1996b).

On the Mejillones Peninsula (northern Chile), which

is located about 60 km east of where the Nazca Plate

impinges on the Peru–Chile subduction zone, segment-

ed N–S-striking normal faults dip to the east and define

two major half-graben (the Pampa Mejillones and

Pampa Caleta Herradura basins) flanked by the Morro

Mejillones and Morro Jorgino–La Rinconada fault seg-

ments, respectively (Fig. 1). These basins have been

filled by a diverse assemblage of Miocene to Pleisto-

cene siliciclastic and carbonate deposits that accumu-

lated in non-marine, marginal marine, and shallow

marine settings. Further extensional tectonics occurred

from the Middle Pleistocene onwards along a complex

NW–SE and NNW–SSE trending en echelon fault sys-

tem (Armijo and Thiele, 1990). This system, which has

disrupted the entire Cenozoic infill of the two major

half-graben, comprises a dense population of active

faults of 100 m to greater than 2.5 km in length and

Fig. 1. Map of the Mejillones Peninsula. (A) Regional tectonic setting of the Mejillones Peninsula area modified by Niemeyer et al. (1996). (B)

Tectonic sketch map of the Mejillones Peninsula showing distribution of main geological features. MMB, Morro Mejillones Block; CMB, Cerro

Moreno Block; MJB, Morro Jorgino Block; MF, Mejillones Fault segment; JF, Jorgino Fault strand; RF, La Rinconada Fault strand; PM, Pampa

Mejillones basin; CH, Caleta Herradura subbasin; CB, Cerro Bandurrias subbasin; PA, Pampa del Aeropuerto subbasin.

G. Cantalamessa et al. / Sedimentary Geology 183 (2006) 125–144 127

exhibits characteristics indicating that they were active

during deposition. Faults, which are oriented roughly

subparallel to the half-graben bounding fault and

obliquely to the inferred palaeoshore trend, have both

antithetic and synthetic dips, although antithetic dips

dominate. They cut the sea-cliff along the coast stretch

to the east of La Rinconada (Fig. 2), and form a series

of closely spaced, meter- to decimeter-scale scarps and

minor intra-basinal grabens, half-graben, and rotated

fault blocks.

The Neogene stratigraphic record varies consider-

ably around the Pampa Caleta Herradura basin and

allows its subdivision into a northern (Caleta Herra-

dura), central (Cerro Bandurrias), and southern (Pampa

del Aeropuerto) subbasin. The lithostratigraphic frame-

work for the Neogene sedimentary infill of this basin

was first established by Krebs et al. (1992) and later

refined by Niemeyer et al. (1996) (Fig. 3). In summary,

underlain by a complex basement composed of Cam-

brian metamorphic, Jurassic granitoids and volcanics,

and Cretaceous sedimentary rocks, it comprises five

Neogene unconformity bounded units: (i) the Miocene

Caleta Herradura Formation; (ii) the Pliocene Cerro

Bandurrias, Cuesta del Burro, and La Portada Forma-

tions; (iii) the Pleistocene Mejillones Formation, the

object of this study. The Cuesta del Burro and Mejil-

lones Formations are laterally equivalent to part of the

Pliocene and Pleistocene marine terraces carved on the

footwall blocks (Niemeyer et al., 1996; Cantalamessa et

al., 2004).

The Morro Jorgino–La Rinconada fault segment is

composed of two smaller fault strands (sensu Roberts

and Gawthorpe, 1995): the Morro Jorgino and La Rin-

conada fault strands. The latter bounds the downfaulted

Pampa del Aeropuerto subbasin, the infill of which is

exposed continuously along the 16 km-long sea-cliff of

the eastern Moreno Bay. At this site, a southerly facing

embayment situated at the southeastern end of the

Mejillones Peninsula, the Mejillones Formation rests

unconformably on the 35 m-thick shallow-marine

deposits of the La Portada Formation. Published bio-

stratigraphic data from the La Portada Formation (Ibar-

aki, 2002) and radiometric dating (U/Th) on molluscan

shells of the Mejillones Formation (Ortlieb et al.,

Fig. 2. Aerial photograph (A) and line drawing (B) of the Moreno Bay and the adjacent Pampa del Aeropuerto plain, showing the major faults of the

secondary system and beach-ridge sets. The height of the sea-cliff diminishes progressively westward as a result of a recent roll-over into the

halfgraben-bounding fault (La Rinconada fault strand). Note that each successively younger beach-ridge set extends less far inland than the older

one and that the series of normal faults of the secondary system are oriented obliquely with respect to the palaeoshore trend (indicated by beach-

ridges) and cut the sea-cliff along the stretch of coast to the east of Las Losas.


1996a) indicate that these units have Late Pliocene and

Middle Pleistocene ages, respectively.

In plan view, the Mejillones Formation is composed

of a series of semiparallel, arcuate, low-relief shelly

beach-ridges. They are tens of meters wide and

hundreds of meters to several kilometers long and

oriented roughly parallel to the present shoreline trend

(Fig. 2). The resulting beach-ridge strandplain is divid-

ed into a number of downstepping sets of beach-ridges

that are separated from each other by major lateral

discontinuities in their geometry and are thought to be

coeval with as many successive interglacial episodes

Fig. 3. Stratigraphic chart summarising the main units of the Pampa Caleta Herradura basin based on: 1—Ortlieb et al. (1996a); 2—Ibaraki (2002);

3—Hartley et al. (1992); 4—Niemeyer et al. (1996); 5—Flint et al. (1986), Flint and Turner (1988); 6—Krebs et al. (1992); 7—Ibaraki (2001).


(Ortlieb et al., 1996a). In particular, Ortlieb et al.

(1996a) ascribed the youngest set of mollusc-bearing

beach-ridges of the Mejillones Formation to the marine

isotope stage (MIS) 9 (c. 330 ky BP). The paucity of

suitable and significant exposures within the older

Fig. 4. Depositional strike-oriented correlation panel of the studied section sh

both at the hanging-wall and footwall of the secondary normal fault syste

continuous exposures of the Moreno Bay coastal-cliff (see Fig. 2 for locatio

main bounding surfaces.

beach-ridge sets, mostly consisting of scattered small

outcrops at the footwall of faults of the secondary

system, precludes the establishment of an accurate stra-

tigraphy for these deposits. However, detailed stratig-

raphy of the MIS 9 beach-ridges provides a useful

owing a synthesis of distribution and architecture of facies associations

m. These simplified stratigraphic sections were measured along the

ns). Lateral correlations were established directly by walking out the


reference for interpreting the internal organisation of

sediments of the older sets.

The study area lies in the coastal zone of the Ata-

cama Desert, one of the major hyper-arid deserts of the

world. A number of sedimentologic data from local

middle Miocene to upper Pliocene successions (Saez

et al., 1999; Hartley and Chong, 2002) suggests that

aridification of this region has been progressive and

primarily induced by the combined influence of a high

atmospheric pressure cell, the presence offshore of the

northward-flowing, cold Humboldt Current, and the

rain-shadow effect of the Andean Cordillera (Houston

and Hartley, 2003). According to Hartley and Chong

(2002), aridification commenced during Miocene with

the establishment of a semiarid climate that, repeatedly

punctuated by relatively shorter, hyper-arid phases,

persisted also during the early Pliocene. The shift into

the present-day hyper-arid climate, which appears to

have remained a constant factor throughout the Quater-

nary climatic fluctuations (Clapperton, 1993), was trig-

gered by a phase of global climate cooling and is

recorded by evaporite precipitation during late Pliocene

and Pleistocene.

3. Description and interpretation of sedimentary

facies associations

At Pampa del Aeropuerto sediments of the Mejil-

lones Formation consist of a variable assemblage of

siliciclastic and richly fossiliferous strata well-exposed

at the top of the sea-cliff. They have been subdivided

into two major facies associations separated by a later-

ally extensive disconformity accompanied by a signif-

icant lithological change and a different sedimentary

style. Extensive exposures on the coastal cliff allow the

stratigraphy to be reconstructed and traced laterally in

great detail (Fig. 4). Component sedimentary facies,

which were deposited in an array of strandline and

shallow-marine settings, have been distinguished on

the basis of field observations including paleontological

content and taphonomic features of shell-bearing inter-

vals, physical sedimentary structures, grain size, lateral

and vertical contacts, and trace fossils.

3.1. Facies Association A (FA-A): lower shoreface

3.1.1. Description

Facies Association A shows an overall upward fin-

ing and thinning of grain size and occurs solely in

hanging-wall sections of the intra-basinal normal faults

where it forms a laterally discontinuous, less than 3 m-

thick wedge (Fig. 4). Its base is typically defined by a

slightly irregular erosional surface that may be either

mantled by a coarse, up to 40 cm-thick shell-pavement

composed of disarticulated, abraded and bio-eroded

molluscan shells, or intermittently blanketed by a one-

clast-thick layer of pebble- to cobble-grade, well-round-

ed clasts of Pliocene provenance (Lithofacies A1) (Fig.

5A). Commonly, the surface is intensely penetrated by

well-preserved, inclined to vertical, I- and J-shaped

cylindrical burrows that descend into the fine-grained

sediments of the underlying La Portada Formation to

depths of as much as 25–30 cm (Fig. 5B). Shafts, which

may be up to 2 cm in diameter, are sharp-walled,

unlined, and passively filled with medium- to coarse-

grained sand piped down from above the discontinuity.

Lithofacies A2 immediately overlies the basal ero-

sional surface and, where present, the basal gravel or

shell lag of Lithofacies A1. It consists of thin to thick

sets of laminated to very low-angle cross-stratified

medium- to granule-grained sandstones, with common

hummocks and parallel-lamination and rare swales pre-

served. Beds are 10–50 cm thick, with tabular or len-

ticular geometry. Normally, bioturbation is present but

is not pervasive. Molluscs are rare and mainly pre-

served as small shell fragments aligned along laminae

(Fig. 5B). Locally, Lithofacies A2 grades upwards into

the significantly finer grained Lithofacies A3. It has a

banded appearance produced by the rhythmical alterna-

tion of centimeter- to decimeter-thick, parallel-laminat-

ed layers of yellow silty sands to very fine sands and

minor intercalations of bluish silty clays. Decimeter-

scale cross-laminations, dipping in along-shore direc-

tions, are present in rare, medium-grained sand layers.

3.1.2. Interpretation

The occurrence of vertical, unlined, and passively

infilled burrows subtending from the basal surface sug-

gests that construction of burrows in the shaly sedi-

ments of La Portada Formation occurred in a substrate

that was firm enough to support such features. This has

important implications regarding the interpretation of

the origin of the surface in question. Firmgrounds in

fine-grained, clastic substrates are typically derived

from exhumation of sediments that have undergone

burial, compaction, and partial dewatering, and may

be demarcated by the Glossifungites ichnofacies, a

substrate-controlled suite of trace fossils strongly indic-

ative of firm but unlithified surfaces (Frey and Seila-

cher, 1980). The Glossifungites ichnofacies reflects

condition at, or soon after, the process of erosional

exhumation and before the onset of sedimentation of

the overlying FA-A (MacEachern et al., 1992b; Pem-

berton et al., 2001). As such, these firmground struc-


tures subtend a discontinuity surface associated with a

depositional hiatus and a pronounced stratigraphic

break in the rock record. The shell lag and the gravel

bed, which is composed of clasts that are thought to

have been sourced from the cutting of the basal uncon-

formity, are interpreted as typical transgressive lags

deposited in a high-energy shoreface by wave-rework-

ing and winnowing during passage of the surf zone, and

left behind as transgression continued.

Alternating beds of parallel-laminated and amalgam-

ated hummocky cross-stratified sandstones (Lithofacies

A2) are interpreted to be deposited by waning of com-

bined oscillatory flow and unidirectional geostrophic

currents, and waning oscillatory flows (Cheel, 1991;

Duke et al., 1991), which are supposed to be triggered

by repeated episodes of storm-driven currents and

waves in a high-energy, shallow marine environment

(e.g. Dott and Bourgeois, 1982; Leckie and Walker,

1982; Plint and Walker, 1987). The lack of mud drapes

further supports the interpretation of deposition in ag-

itated-water conditions in a lower shoreface setting.

Lithofacies A3 is inferred to represent distal shoreface

to transition deposition at depths close to the fair-

weather wave base.

Fig. 5. Outcrop photographs showing various sedimentological characteristics of facies and Facies Associations within the studied units. (A)

Pebble- to cobble-grade conglomerates along the base of FA-A (see white arrows) representing a transgressive lag. (B) The white arrow points to the

abundant shafts of the Glossifungites ichnofacies subtending from the surface interpreted as the RS coplanar with the sequence boundary. The RS

removed any evidence of previous subaerial exposure. Note the vertical, sharp-walled and unlined nature of the traces indicating the firmness of the

substrate at the time of burrow excavation. Tube diameters reach 1.5 cm. A laterally discontinuous pebble- to boulder-grade lag occurs along the

contact elsewhere. (C) Strike-oriented view of the abrupt contact between sediments of La Portada Formation and the overlying FA-B. Note the

presence of a series of 10–20 cm deep and 30–50 cm wide gutter casts along the base of FA-B. Hammer for scale. (D) Close view of the contact

between FA-A and FA-B showing the presence of a discontinuous lag of angular, ripped up sandstone intraclasts. Note the coarse and tightly packed

mollusc skeletons that characterise the lower part of FA-B. (E) Enlargement of (D) showing detail of angular sandstone clasts occurring along the

lower part of FA-B. They are encrusted by barnacles on the lower side (white arrows). This position of barnacles is untenable for life (their apertures

would have been forced into the sediment), implying overturning of the clasts after the barnacles grew on the upper surface. Lens cap for scale. (F)

Close view of a subangular clast sourced from the La Portada Formation with bivalve internal moulds and encrusting barnacles (black arrow). (G)

Close view of a section roughly normal to palaeo-shoreline. Note the sharp lithologic contrast between FA-A (lower half of photo) and the mollusc-

bearing, seaward dipping (to the left in the photo) cross-beds of FA-B (upper half). Note also the isolated tongues of onshore dipping planar cross-

stratified sands and the erosional truncation separating Lithofacies B1 and B3 in the middle part of FA-B. Dashed line indicates the SB/RS. (H and I)

Outcrop photographs showing the thin and regular stratification of Lithofacies B3, and the segregation of bioclasts of different sizes into separate

beds. Hammer and lens cap for scale, respectively. (J) Close view of Lithofacies B3. Note the openwork fabric of shell debris and the rounded edges

of shell fragments.


Fig. 6. Outcrop photographs of (A) a hanging-wall section (measured section 4 in Fig. 4) and (B) a footwall section (measured section 5 in Fig. 4).

At the hangingwall of normal faults the Mejillones Formation contains both the transgressive and falling-stage systems tracts and the lower

sequence boundary consists of a transgressively modified surface of subaerial exposure (SB/RS). At the footwall only the falling-stage systems tract

is preserved and the sequence is bounded below by a regressive surface of marine erosion superimposed upon the previous transgressively modified

sequence boundary (SB/RS/RSME). About 6 m and 11 m of section are shown in (A and B), respectively.


3.2. Facies Association B (FA-B): beachface

3.2.1. Description

Facies Association B displays a general fining-up-

ward trend. It rests disconformably on sediments of

FA-A at the hanging-wall of the intra-basinal normal

faults, but overlies unconformably sediments of La

Portada Formation on footwall highs, where sediments

of FA-A are missing (Fig. 4). On the outcrop scale, the

basal contact of FA-B is typically sharp and erosional,

laterally extensive, and essentially flat. However, on a

more detailed scale, it is an irregular, scalloped surface

locally marked by erosional scours up to 40 cm deep

and 70 cm wide (Fig. 5C). Scours, which are essen-

tially parallel to each other and orientated roughly

perpendicular to the palaeo-shoreline trend, are lateral-

ly irregularly spaced and infilled with whole and frag-

mented shells. No substrate-controlled ichnological

suites were observed to demarcate this surface.

Aligned along the basal contact or, to a lesser extent,

included in the lowermost part of this lithofacies, a

discontinuous lag of dispersed clasts ranging in size

from granule to cobbles or boulder occurs (Fig. 5D).

Clast composition reflects an intrabasinal source. They

consist both of angular to subangular, partially lithified

sandstone intraclasts ripped-up from the Facies Asso-

ciation A (in places encrusted by small barnacles) (Fig.

5E, F), and rounded to subangular, bored clasts of

sediments of the immediately underlying La Portada

Formation, with the latter significantly most numerous

on footwall crests. This facies association comprises

three distinct lithofacies characterised by significantly

different textural features, stratification types and av-

erage dip angle of the beds, and reflecting two different

subenvironments. Its thickness in strike direction is

variable and ranges from about 7 m on footwall

highs to about 10 m in hanging-wall lows. Lithofacies

B1, forming the bulk of the lower part of the associ-

ation, is commonly thinner in hanging-wall sections. It

is composed mostly of well-sorted, bioclast-supported

whole shells with matrix of coarse sand, and charac-

terised by high-angle, seaward-dipping, irregular and


poorly distinct, faint planar cross-bedding (Fig. 5G).

The mollusc assemblage is overwhelmingly dominated

by adult and disarticulated specimens of the infaunal

bivalve Mulinia cf. M. edulis displaying mainly a

concave-up orientation, and less abundant members

of the gastropods Concholepas concholepas and

Oliva peruviana, and the bivalve Argopecten purpur-

atus. The largest shells within individual beds of this

cross-stratified biocalcirudite tend to be concentrated

near the base, where they are more tightly packed and

chiefly oriented concave-upward. Shells, which de-

crease significantly in size upward along the cross-

Fig. 7. (A) Photograph and (B) line drawing of an outcrop view east of La

encountered across the normal faults. About 10 m of section oriented normal

about 5 m. Box in the left side indicates the position of Fig. 5A. The complet

is preserved in hanging-wall sections (to the left of the geologist), whereas o

of the geologist). Note the irregular morphology of the RSME at the base of

high subsidence of the hanging-wall, also the transgressive deposits of an o

stratification, are heavily abraded, moderately fragmen-

ted, nested, little affected by biological processes

(bioerosion and encrustation) and often imbricated.

Bioturbation is rarely observed. Locally, isolated ton-

gues of onshore dipping planar and trough cross-strat-

ified sands (Lithofacies B2) occur in the lower part of

this Facies Association. These tongues, generally 15–

30 cm thick, interfinger with shell layers of Lithofacies

B1 and wedge out landwards (Fig. 5G). The vertical

transition between Lithofacies B1 and B3 is realised by

means of surfaces that truncate the underlying beds at

low angles (Fig. 5G).

s Losas, showing the abrupt lateral variation in sequence architecture

to depositional dip is shown. Relative displacement across the fault is

e transgressive–regressive architecture of the studied sequence (MIS 9)

nly its regressive portion is preserved in footwall sections (to the right

the FSST. Geologist in the centre for scale. In this case, owing to the

lder depositional sequence (MIS 11?) is preserved.


Lithofacies B3, forming the upper part of the asso-

ciation, shows a seaward-dipping, low-angle planar

cross-bedding that may readily be distinguished from

that of the Lithofacies B1. Beds consist of thin, closely

packed and well-sorted layers of very coarse sand- to

granule-grade shell fragments with good to excellent

segregation of bioclasts of different sizes into separate

beds alternating with less numerous layers of coarse-

grained, shelly lithic sands (Fig. 5H, I). Shell debris is

dominantly clast-supported, with some layers showing

an openwork fabric (Fig. 5J). Although shells are the

conspicuous feature of this lithofacies, bioturbation is

rare to absent and restricted to vertical escape burrows.

Fauna is largely dominated by the bivalves Mesodesma

donacium and Mulinia cf. M. edulis, which show a

balanced proportion of right and left valves. Much less

abundant taxa include specimens of the bivalve A.

purpuratus and the gastropod O. peruviana. Both

whole valves and shell fragments show rounded

edges, extensive abrasion, and may retain borings com-

Fig. 8. Generalised model (not to scale) illustrating the possible sequence o

distribution. The relative sea-level oscillation and the component architectu

monly abraded at their openings. Whole shells display

variable spatial disposition, as both concave-upward

and concave-downward orientations occur. Encrusta-

tion is rare to absent.

3.2.2. Interpretation

Interpretation of this shell-bearing, regressive wedge

is facilitated by the fact that in plan view it forms

laterally continuous beach-ridges forming an extensive

raised strandplain (Fig. 2). As pointed out by Massari

and Parea (1988), who described in detail a lithoclast-

bearing analogue for this biocalcarenite to biocalciru-

dite Facies Association, the three lithofacies may rep-

resent the sedimentary record of the lower (Lithofacies

B1 and Lithofacies B2) and upper beachface (Lithofa-

cies B3). These are defined as the submarine sloping

face of the beach and that part of the beach that extends

updip from the foreshore to the highest berm, respec-

tively (Bourgeois and Leithold, 1984). Particularly, the

imbricate, seaward-dipping planar cross-stratified strata

f events in the origin of the observed along-strike variation in facies

ral elements are shown within the inset. See text for discussion.


are regarded as reflecting accretion of the beachface

during progradation, the onshore-dipping sand tongues

interstratified with them are interpreted as the product

of the landward migration of bars during periodic re-

covery stages, and the erosional features interposed

between Lithofacies B1 and B3 as the result of trunca-

tion during high-energy events. This depositional set-

ting, along with a low siliciclastic sediment supply,

resulted in prolonged exposure of shells and shell frag-

Fig. 9. Generalised depositional dip-oriented sketch (no scale implied) illus

bounding surfaces down the axis of the gulf. The overall progradation of the

and intervening landward shifts of shoreline controlled by glacio-eustatic, hi

term sea-level fall. Sequences (beach-ridge sets), which may have been part

erosional ravinement during the next transgression, are stacked in an downste

falling-stage sequence set. Boundaries of older sequences, which formed dur

level rise and erosional reworking, are truncated downdip by the successive e

below by a complex surface that develops diachronously from the down-di

ments on the sea-floor and, therefore, in their repeated

wave-reworking and intense abrasion.

The paucispecific composition of the mollusc as-

semblage (i.e. the dominance of few taxa in the mollusc

assemblage; Kidwell et al., 1986) deserves special

mention. Indeed, it seems that paucispecific or mono-

specific assemblages are the norm rather than an ex-

ception in Quaternary beach-ridge concentrations (e.g.

Kowalewski et al., 1994). The taxonomic composition

trating a conceptual model for the development of systems tracts and

Pampa del Aeropuerto strandplain occurred through forced regressions

gh-frequency oscillations superimposed on a tectonically driven long-

ially destroyed by subaerial processes during the emergence stage and

pping configuration and, as a whole, represent a tectonically enhanced,

ing sea-level fall, sub-aerial exposure of the shelf and subsequent sea-

rosion surface. Consequently, the falling-stage sequence set is bounded

p merging of successive composite sequence boundaries.


of shelly beach-ridges may be either significantly bi-

ased and to reflect purely taphonomic phenomena, such

as size and shape segregation of shells by hydraulic

sorting (Meldhal, 1993) and preferential preservation of

taxa more resistant to mechanical fragmentation (Kid-

well and Bosence, 1991), or controlled by occurrences

of dense populations of shelly taxa that have high

turnover rate (i.e. opportunistic species with high fe-

cundity, early sexual maturity, rapid growth, and high

mortality rate) and, as a result, may contribute an

enormous quantity of shells to death assemblages

(Kowalewski et al., 1994).

To some extent, the erosional scours present on the

base of the FA-B bear strong resemblance to structures

observed at the base of erosive-based shoreface sand

bodies and termed gutter casts (e.g. Plint, 1988, 1996).

The absence of burrowing along the entire extension of

the basal surface suggests that this surface is not asso-

ciated with a significant depositional hiatus and that

erosion of the underlying sediments was immediately

followed by, if not synchronous with, deposition of the

preserved cover. Additionally, the occurrence of ripped

up, intraformationally derived material supports a ge-

netic affinity between erosion of the seafloor and em-

placement of Lithofacies B1 (i.e. these clasts have been

eroded at the surface at the base of FA-B and rapidly

incorporated into its basal portion). The presence of a

number of coarse-grained sandstone intraclasts implies

that the eroded material may have been subject to a

degree of compaction and early diagenesis.

4. Sequence-stratigraphic framework

The sequence stratigraphic model developed for the

Mejillones Formation is based upon the recognition of

key stratal surfaces, facies shifts, and facies stacking

patterns derived from detailed outcrop-based interpre-

tation. Component sequences display a variety of types

of facies and chronostratigraphically significant key

erosion surfaces that can be easily recognised in out-

crop. The internal facies architecture of sequences,

combined with position and nature of bounding discon-

tinuities, provides the basis for establishing a high-

resolution sequence stratigraphic framework. In terms

of sequence stratigraphy, the diverse assemblage of

lithofacies documented in the previous sections can

be attributed to the glacio-eustatic sea-level oscillation

occurred during MIS 9 (Ortlieb et al., 1996a) and, as a

such, it represents a 5th-order (sensu Fulthorpe, 1991),

100-ky-long depositional sequence. Albeit incomplete,

the sequence can be divided into a transgressive sys-

tems tract (TST) and a falling-stage systems tract

(FSST), which have been erected according to the

four-fold systems tract division proposed by Plint and

Nummedal (2000) (i.e. the upper sequence boundary is

the subaerial unconformity located at the top of the

falling-stage systems tract). The key stratal surfaces

and the intervening stratal units are described in the

order they are commonly developed, commencing with

the lowermost surface.

In the study area, deposits attributable to the low-

stand systems tract (LST) have not been observed,

because of the geographic position of the outcrops,

which are located several kilometers landward of the

contemporaneous shelf edge. In the inner part of the

basin, periods of glacial lowstand are represented by

erosional surfaces that mark sequence boundaries and

subaerial exposure.

Because the Glossifungites ichnofacies is capable of

demarcating transgressively modified sequence bound-

aries (MacEachern et al., 1992a; Pemberton and

MacEachern, 1995), the erosional, Glossifungites-de-

marcated surface at the base of Facies Association A

is interpreted as a transgressive surface of erosion

(ravinement surface, RS). Ravinement occurs by wave

erosion in the upper shoreface/nearshore zone of the

surf during phases of erosional shoreface retreat (Num-

medal and Swift, 1987). This surface is superimposed

on a previous subaerial sequence boundary (SB), form-

ing a composite transgressive surface and sequence

boundary (SB/RS) lacking of any evidence of subaerial

exposure. Owing to its fining- and deepening-upward

trend, the overlying Facies Association A is interpreted

as the backstepping shelf wedge developed during the

successive landward migration of the shoreline and,

therefore, to represent the deposition of the transgres-

sive systems tract (TST). The contemporaneous upper

shoreface and beach deposits are not preserved here,

and are interpreted to have been eroded during shore-

face retreat and development of the ravinement surface.

The highstand systems tract (HST) develops during

relative sea-level rise when rates of sediment supply are

high enough to outpace rates of relative sea-level rise.

The exposures of the Mejillones Formation along the

Moreno Bay coastal-cliff lack deposits assignable to the

HST. It therefore is possible that in this area the HST

was either never developed owing to the low sediment

supply related to the hyper-arid climate of the region

during Quaternary (Hartley and Chong, 2002) and the

consequent absence of fluvial runoff, or was extremely

thin and eroded together with the upper part of the TST

by wave scouring during relative sea-level fall. The

maximum flooding surface (MFS) indicates the maxi-

mum landward extension of shoreline during transgres-


sion and is commonly used to mark the boundary from

TST deposits below and HST deposits above. In the

Mejillones Formation, the maximum marine transgres-

sion within individual sequences is indicated by the

major discontinuities in beach-ridges geometry visible

in plan view. In outcrop sections, however, clear phys-

ical indicators of this surface have not been observed.

This may be due either to the general absence of

physical expression of this surface at outcrop scale

(Carter et al., 1998) or to the absence of deposits that

might be related to the HST. In the latter case the MFS

could have been easily cut and superimposed by the

erosional surface at the base of the falling-stage systems

tract.

The falling-stage systems tract (FSST) forms during

relative sea-level fall (Plint and Nummedal, 2000 and

references therein). Recognition of this stratal unit is

based on a series of criteria (Plint, 1988; Plint and

Nummedal, 2000), two of which have been identified

in the study area: (i) the downstepping migration path

of the shoreline along the depositional dip; and (ii) the

occurrence, on a sharp, gutter-casted surface, of a shal-

lowing-upward shallow-marine succession marking an

abrupt basinward shift in facies. At Pampa del Aero-

puerto, within a single set of beach-ridges, progressive-

ly younger beach-ridges are encountered at lower

altitude from landward to seaward locations (i.e. from

north to south). Such a migration pattern is a clear

evidence that beach-ridge progradation has taken

place during a relative sea-level fall and supports the

occurrence of forced regression. In addition, bedding

planes in the downstepping wedge downlap onto an

erosional surface interpreted as a regressive surface of

marine erosion (RSME) that is formed by wave scour-

ing in front of an advancing shoreline (Plint, 1988;

Nummedal et al., 1993). As such, this erosional surface,

which separates the transgressive shoreface sediments

(FA-A) beneath from the forced regressive beachface

deposits (FA-B) above, is good evidence for a basin-

ward shift of facies and coastal depocentres, and pre-

dicts subaerial exposure and development of a sequence

boundary in more proximal areas. At such times, waves

were able to erode early lithified transgressive sand-

stones to form an intraclast regressive lag. The empty-

ing of coastal sediment traps associated with sea-level

drop should greatly increase the influx of sediments

into shoreline settings and to the contemporaneous

FSST. However, the low rate of terrigenous sedimenta-

tion required for the accumulation of the carbonate-rich

FSST described above suggests that in this instance

forced regression occurred under conditions of signifi-

cantly low siliciclastic supply, perhaps because of the

contemporaneous hyper-arid climate. Deposition of

beach-ridges during phases of forced regression is

also proven by the absolute age of these sediments.

Indeed, results from U/Th determinations on mollusc

shells collected in the Facies Association B yielded

absolute ages ranging from 275F11 ky to 288F12

ky (Ortlieb et al., 1996a), which indicate that their

deposition has taken place during the falling limb of

MIS 9.

Quaternary sediments laid down during periods of

continuously falling sea-level are widely recognized

both from analyses of high-resolution seismic reflection

profiles of continental margins (e.g. Chiocci, 2000;

Hernandez-Molina et al., 2000; Trincardi and Correg-

giari, 2000), and outcrop-based studies (Massari, 1997;

Massari et al., 1999; Tropeano and Sabato, 2000; Can-

talamessa and Di Celma, 2004).

The upper sequence bounding unconformity is still

evolving today and corresponds to the present surface

of subaerial exposure.

5. Discussion

5.1. Origin of depositional sequences

When the magnitude and time-scale of the eustatic

signal is known by independent sources such as the

marine oxygen–isotope curve (e.g. Shackleton et al.,

1990), it is possible to discriminate the eustatic from the

tectonic component of the relative sea-level change by

comparing the rate of glacio-eustatic sea-level change

and the rate of tectonic uplift/subsidence. Earth’s cli-

matic history during the Quaternary has been controlled

by Milankovitch variations in planetary orbit, compris-

ing alternate periods of glaciation and interglaciation.

The waxing and waning of ice sheets produced changes

in the volume of water in the global ocean so that high-

frequency, high-amplitude sea-level fluctuations domi-

nated by obliquity-driven, 41-ky-long cycles in the

early Pleistocene (Shackleton et al., 1990) and eccen-

tricity-driven, 100-ky-long cycles in the middle–late

Pleistocene (Bassinot et al., 1994) occurred. Eccentric-

ity-driven cycles were characterised by a distinct asym-

metry between the rising (average rate of c. 10–15 m

ky�1) and falling (average rate of c. 1–1.5 m ky�1)

limbs with about 80% of time spent in eustatic fall.

Therefore, as the rate of middle–late Pleistocene eustat-

ic sea-level changes were sufficiently high to outpace

the rate of regional tectonic uplift recorded in the study

area, eustasy triggered by astronomically induced cli-

matic changes could have effectively controlled the

development of the sets of beach-ridges observed at


Pampa del Aeropuerto. The resulting shallow marine

sedimentary record is cyclic, with each beach-ridge set

corresponding to a single variation in relative sea-level

and must therefore be termed depositional sequence.

The landward and seaward shifts of the shoreline, as

well as the timing of systems tracts and key surfaces,

are controlled by the interplay between supply regime

and rates of relative sea-level changes. Owing to the

hyper-arid climate of the Mejillones region during the

Pleistocene, fluvial discharge was virtually absent and

the rate of siliciclastic sediment supply to shorelines

was negligible. Such a condition, which created a basin

essentially starved of clastic sediment, is clearly

reflected in the absence of the sediment-driven HST,

in the overall reduced thickness of depositional

sequences, and in the carbonate-rich FSST.

5.2. Fault controls on sequence architecture

Sequence stratigraphy provides a framework within

which aspects of local and global sea-level fluctuations,

tectonics and subsidence, patterns of sedimentation, and

dynamics of sedimentary basin fill can be examined.

The sequence stratigraphic interpretation described

above allows further constraints to be placed on fault

controls on sequence architecture and nature of bound-

ing surfaces. Indeed, the observed systematic changes

of sequence features across normal faults (Figs. 4 and

6) suggest that syn-depositional block faulting, through

its control on local development or loss in accommo-

dation, must have had a primary role in determining the

preservation potential and the overall distribution of

transgressive deposits, and the genetic complexity of

bounding surfaces. If so, to allow for the two markedly

different architectures observed, short-term accommo-

dation space must have varied significantly during se-

quence deposition. Accommodation is the space,

created by eustatic sea-level rise, subsidence, or a com-

bination of these two main parameters, into which

sediments may be deposited. Because variation on the

rate of accommodation development is a function of the

rate of sea-level change (Jervey, 1988), accommodation

development or loss can be considered as a function of

both eustatic sea-level change and subsidence/uplift. At

Pampa del Aeropuerto the tectonic component of ac-

commodation results from the interplay of the (i) basin

subsidence, (ii) short-term, hanging-wall subsidence

and footwall uplift of the small-scale normal faults,

(iii) the longer-term regional uplift that affect both the

hanging-walls and footwalls.

The hanging-wall vertical separation and footwall

superimposition of the transgressive RS and its regres-

sive counterpart, the RSME, indicate that footwalls rose

and hanging-walls subsided between the formation of

these two surfaces. On top of footwall blocks, owing to

the their uplift, accommodation is lost and the overall

preservation potential of transgressive deposits is low

as they are more prone to erosion during relative sea-

level fall. In the studied sequence, transgressive sedi-

ments on upthrown blocks have been completely re-

moved by wave erosion during sea-level lowering. In

addition, the absence of the underlying Glossifungites-

demarcated RS provides evidence which suggest that

also a substantial erosion of the underlying Pliocene

deposits occurred and that wave erosion cut as deeply

as 40 cm into underlying sediments. Hanging-wall

subsidence, inversely, created new accommodation

space and allowed the preservation of transgressive

deposits on downthrown blocks.

All of these features are interpreted to indicate that

the development of normal fault zones was a primary

control on the stratigraphy of the Mejillones Formation

and led to marked spatial variability in systems tracts

and key stratal surfaces. Furthermore, the accommoda-

tion development varied markedly between the fault

zones, so that within the more rapidly subsiding hang-

ing-walls the TST of more than a single depositional

sequence was likely be preserved (Figs. 4 and 7).

Fig. 8 shows a model that may help to explain

variations in sequence architecture across normal faults

in response to locally varying accommodation. At time

T1, during a rapid sea-level rise and landward migra-

tion of coastline, the subaerial surface of exposure (SB)

was eroded and replaced by a Glossifungites-demarcat-

ed RS (coplanar SB/RS) covered by a wave-winnowed,

transgressive conglomerate or shell lag. At T2, as trans-

gression proceeded, new accommodation was created,

more sediment was added to the rock record, and a

relatively thin TST was deposited. At such time, incre-

mental fault movement resulted in uplift of footwall

blocks and subsidence of hanging-wall blocks. Exten-

sional subsidence rates exceeded the contemporaneous

regional uplift rates so that hanging-wall undergone net

subsidence. During the succeeding longer-lasting phase

of sea-level fall, accommodation was progressively

destroyed in response to sea-level lowering. Wave

scouring in front of the advancing shoreline cut and

smoothed the antecedent fault-generated sea-floor mor-

phology and the poorly consolidated, recently deposited

sediments were quickly eroded, leaving an intraclast lag

on the unconformity surface (RSME) (T3). Under these

circumstances, depending on the hanging-wall or foot-

wall setting, two different sequence architectures were

generated. At the hanging-walls, because of net subsi-


dence, wave scouring attained a position that was rel-

atively shallower than that reached during transgres-

sion, so that only part of the TST was eroded during

formation of the RSME. At the same time, the positive

surface relief of footwall highs starts being eroded,

reworked, and removed by wave scouring until the

floor was reduced to the same level as that in the

hanging-walls. However, because of the relative uplift

of footwall blocks, this position was deeper than that

reached during transgression. Consequently, the entire

TST and part of the underlying Pliocene bedrock were

eroded from the crest of footwalls, forming a tectoni-

cally enhanced (Gawthorpe et al., 2000), coplanar re-

gressive surface of marine erosion and sequence

boundary (SB/RS/RSME). As a result, in hanging-

wall sections the FSST directly overlies the TST,

whereas in footwall sections the FSST rests directly

on Pliocene sediments of the La Portada Formation.

At T4, beachface deposits filled the new accommoda-

tion created landward of the contemporary RSME and

prograded.

A sequence stratigraphic architecture remarkably

similar to that observed in the Mejillones Formation

has been documented in the Miocene Lower Rudeis

Formation of the Suez Rift, Egypt (Jackson et al.,

2005). In this setting, activity on basin bounding nor-

mal faults led to hanging-wall preservation of complete

sequences and removal of TST–HSTs from the foot-

wall, where FSSTs are amalgamated and bounded by

composite surfaces.

5.3. Influence of the long-term tectonic uplift on

sequence stacking pattern

Although hanging-wall preservation of TST and,

hence, lateral variation of internal architecture of de-

positional sequences was strongly influenced by move-

ments along local faults, a more regional control on

accommodation influenced the overall sequence stak-

ing pattern within the Mejillones Formation. At least

eight beach-ridge sets (i.e. depositional sequences)

were recognised in the Pampa del Aeropuerto (see

Fig. 2). The long-term effects of the regional tectonic

uplift on these sequences can be evaluated by exam-

ining their vertical arrangement. The major lateral dis-

continuities in geometry of beach-ridges, which record

the point attained by shoreline transgression during

each interglacial maxima, are encountered at lower

altitude from landward to seaward locations, reflecting

both the progressively farther basinward and down-

ward displacement of successive maximum shorelines

through time, and the downstepping stacking pattern of

depositional sequences. Thus, as a whole, the series of

beach-ridge sets of Pampa del Aeropuerto is thought to

represent a composite, northward-offlapping falling-

stage sequence set (Mitchum and Van Wagoner,

1991) formed through a combination of fluctuations

in sea-level resulting from the Quaternary glaciations

superimposed on moderate regional uplift. Similar low-

order composite sequences have been described by

Gawthorpe et al. (1994), Jones and Milton (1994),

Milli (1997), Plint and Nummedal (2000), Hamberg

and Nielsen (2000), and Cantalamessa and Di Celma

(2004). Such a sequence set is underlain by a compos-

ite regressive surface of marine erosion formed by the

lateral connection of lower-rank, seaward-dipping se-

quence boundaries of various nature and that includes

both SB/RSs and SB/RS/RSMEs (Fig. 9).

The long-term trend of relative sea-level fall that has

driven forced regression is a product of regional tec-

tonic uplift.

6. Conclusions

The Mejillones Formation, northern Chile, was de-

posited while the Pampa del Aeropuerto subbasin was

intermittently flooded by the Pacific Ocean and affected

by an extensional tectonic regime superimposed upon

regional tectonic uplift. Sedimentological and sequence

stratigraphic analysis highlights a complex interaction

between low sediment supply, glacio-eustatic sea-level

changes, regional uplift, and local fault movements,

providing insight into the main relationships between

sequence architecture and stacking pattern, syn-deposi-

tional tectonics, and eustatic sea-level fluctuations. The

results stemming from these mid-Pleistocene deposi-

tional sequences have important implications for se-

quence stratigraphic models and leads to the

following general conclusions:

(1) The depositional sequence related to marine iso-

tope stage 9 of the Mejillones Formation can be

divided into siliciclastic lower shoreface and mol-

lusc-bearing beachface Facies Associations,

which also correspond to the transgressive sys-

tems tract and falling-stage systems tract, respec-

tively. Deposits of the transgressive systems tract

are preserved solely at the hanging-wall of the

normal faults of the secondary system. Erosively

based falling-stage deposits, preserved both at the

hanging-wall and footwall of these faults, are the

dominant component of the studied sequence. In

plan view they are represented by a series of

southward facing, arcuate beach-ridges. Progres-


sively younger beach-ridges are encountered fur-

ther seaward and at lower altitude, documenting

downward and southward migration of the

strandline in response to an eustatically driven

sea-level fall.

(2) Although the development of individual deposi-

tional sequences seems to be mainly controlled

by eustasy triggered by mid-Pleistocene astro-

nomically induced climatic changes, the geome-

tries and vertical arrangements of Facies

Associations described from the Moreno Bay out-

crops provide convincing evidence that local de-

formation complicates the internal organisation of

the depositional sequence by adding or subtract-

ing space for sedimentation through tectonic sub-

sidence or uplift, respectively. Indeed, the

systematic along-strike variations of architectural

elements, such as sequence systems tracts and the

nature of key bounding surfaces encountered

across these faults, were driven by short-term

changes in accommodation space that, in turn,

were strongly controlled by local extensional tec-

tonics. A major effect of these dynamics is that

transgressive deposits are more easily preserved

by wave erosion in the hanging-wall of the minor

faults where new accommodation was created by

slip on the minor faults. At these sites the se-

quence boundary is a ravinement surface cut by

waves in the upper shoreface and superimposed

on the subaerial unconformity. The stratigraphic

architecture of the depositional sequence on foot-

wall highs differs markedly from that seen in

hanging-wall lows. Here, footwall uplift com-

bined with marine regressive erosion during rel-

ative sea-level fall led to the complete removal of

the transgressive systems tract and of part of the

underlying Pliocene deposits and the falling-stage

systems tract represents the entire depositional

sequence. At these locations the basal sequence

boundary is a tectonically enhanced erosion sur-

face resulting from the superimposition of the

regressive surface of marine erosion on to the

previous transgressively modified surface of sub-

aerial exposure.

(3) A crustal, large-scale uplift motion, combined

with more local tectonic processes and glacio-

eustasy, drove the progressive shallowing of the

basin. The major influence of regional uplift on

stratal accumulation is on longer term, at sequence

set scale and is responsible for sequence arrange-

ment. The progressive southward and downward

migration in maximum shoreline position of suc-

cessive sequences indicates that each sequence

marks a successive basinward step in the overall

regressive pattern of the Mejillones Formation.

This downstepping arrangement is consonant

with the progressive loss of accommodation due

to the long-term, tectonically driven relative sea-

level fall. High-frequency sequences are nested

within a falling-stage sequence set that is bounded

beneath by a composite surface that develops

diachronously and results from the down-dip

merging of successive sequence boundaries.

Acknowledgements

The manuscript has benefited from advice of J.A.

MacEachern and K.L. Bann on some ichnological

aspects. We also express our gratitude to S.T. Abbott

and an anonymous reviewer for their constructive

remarks as journal referees and to A.D. Miall for the

thorough editing. Financial support was provided by

the Italian Ministry of University and Scientific Re-

search, grant 40% (2002–2004).

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