Unusual coarse, clastic, wave-dominated shoreface deposits, pliocene to middle pleistocene, northern Chile: Implications for coastal facies analysis

Adrian J. Hartley, Elizabeth J. Jolley

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30' S, 70° 14' W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A15 m x 2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four faciès: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface faciès predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by fades analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Mediumscale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

Original languageEnglish
Pages (from-to)105-114
Number of pages10
JournalJournal of Sedimentary Research
Volume69
Issue number1
DOIs
Publication statusPublished - 1 Jan 1999

Keywords

  • HUMMOCKY CROSS-STRATIFICATION
  • RIP CURRENTS
  • SURF ZONE
  • NEARSHORE
  • SYSTEMS
  • BEACH
  • BAR
  • CALIFORNIA
  • SEQUENCES
  • SHELF

ASJC Scopus subject areas

  • Geology

Cite this

@article{c8b0ad2e2c58484d98a9774215e325e2,
title = "Unusual coarse, clastic, wave-dominated shoreface deposits, pliocene to middle pleistocene, northern Chile: Implications for coastal facies analysis",
abstract = "Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30' S, 70° 14' W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A15 m x 2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four faci{\`e}s: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface faci{\`e}s predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by fades analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Mediumscale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.",
keywords = "HUMMOCKY CROSS-STRATIFICATION, RIP CURRENTS, SURF ZONE, NEARSHORE, SYSTEMS, BEACH, BAR, CALIFORNIA, SEQUENCES, SHELF",
author = "Hartley, {Adrian J.} and Jolley, {Elizabeth J.}",
note = "Support was provided by the Nuffield Foundation, the University of Aberdeen, Mobil North Sea Ltd. (AJH), and the Natural Environmental Research Council (EJJ). Grateful thanks to P. Scanlon, T. Buddin, and S. Kape for help in the field and P. Turner for discussion of results. The comments of R.W. Dalrymple, H.E. Clifton, and Associate Editor P.M. Myrow considerably improved the manuscript. This paper is dedicated to the memory of Dr. Phil Scanlon.",
year = "1999",
month = "1",
day = "1",
doi = "10.2110/jsr.69.104",
language = "English",
volume = "69",
pages = "105--114",
journal = "Journal of Sedimentary Research",
issn = "1527-1404",
publisher = "SEPM Society for Sedimentary Geology",
number = "1",

}

TY - JOUR

T1 - Unusual coarse, clastic, wave-dominated shoreface deposits, pliocene to middle pleistocene, northern Chile

T2 - Implications for coastal facies analysis

AU - Hartley, Adrian J.

AU - Jolley, Elizabeth J.

N1 - Support was provided by the Nuffield Foundation, the University of Aberdeen, Mobil North Sea Ltd. (AJH), and the Natural Environmental Research Council (EJJ). Grateful thanks to P. Scanlon, T. Buddin, and S. Kape for help in the field and P. Turner for discussion of results. The comments of R.W. Dalrymple, H.E. Clifton, and Associate Editor P.M. Myrow considerably improved the manuscript. This paper is dedicated to the memory of Dr. Phil Scanlon.

PY - 1999/1/1

Y1 - 1999/1/1

N2 - Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30' S, 70° 14' W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A15 m x 2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four faciès: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface faciès predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by fades analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Mediumscale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

AB - Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30' S, 70° 14' W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A15 m x 2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four faciès: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface faciès predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by fades analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Mediumscale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

KW - HUMMOCKY CROSS-STRATIFICATION

KW - RIP CURRENTS

KW - SURF ZONE

KW - NEARSHORE

KW - SYSTEMS

KW - BEACH

KW - BAR

KW - CALIFORNIA

KW - SEQUENCES

KW - SHELF

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U2 - 10.2110/jsr.69.104

DO - 10.2110/jsr.69.104

M3 - Article

VL - 69

SP - 105

EP - 114

JO - Journal of Sedimentary Research

JF - Journal of Sedimentary Research

SN - 1527-1404

IS - 1

ER -