Chronologic model and transgressive-regressive signatures in the late neocene siliciclastic foundation (long key formation) of the Florida keys

Laura A. Guertin, Donald F. McNeill, Barbara H. Lidz, Kevin J. Cunningham

Research output: Contribution to journalArticle

Abstract

Recent drilling of continuous cores in southernmost Florida has documented a thick unit of upper Neogene siliciclastics subjacent to surficial shallow-water Quaternary carbonates exposed on islands of the Florida Keys. The siliciclastics comprise the Long Key Formation and were identified in two cores collected from the middle and upper Florida Keys. Achronologic model based on new planktic foraminiferal biochronology and strontium-isotope chronology suggests the timing of siliddastic deposition and provides a basis for regional correlation. The chronologic model, supplemented by vertical trends in quartz grain size, pattern of planktic menardiiform coiling direction, and paleoenvironmental interpretations of benthic foraminiferal assemblages, shows that the Long Key Formation contains three intervals (I-III) of varying thickness, grain-size composition, and paleo-water depth. Interval I is uppermost Miocene. The quartz grains in Interval I fine upward from basal very coarse sand to fine and very fine sand. Benthic foraminifera indicate an upward shift from an outershelf to inner-shelf depositional environment. Interval II, deposited during the late early to early late Pliocene, contains reworked upper Miocene siliciclastics and faunas. In the upper Keys, quartz grains in Interval II range from very coarse sand that fines upward to very fine sand and then coarsens to very coarse and medium sand. In situ benthic faunas indicate an upward shift from outer-shelf to inner-shelf deposition. In the middle Keys, Interval II is different, with the quartz grains ranging primarily from medium to very fine sand. In situ benthic taxa indicate deposition on an inner shelf. In both the middle and upper Keys, the upper Pliocene siliciclastics of Interval III contain quartz grains ranging from very coarse to very fine sands that were deposited on an inner shelf. A sequence boundary between Interval I and Interval II is suggested by: an abrupt shift in the strontium-isotope chemostratigraphy; coarsening in quartz grain size above the boundary; an abrupt landward shift in depositional faciès in the upper Keys core; and a distinct variation in the predominant coiling direction of the menardiiform planktic foraminifera, from fluctuating dextral-sinistral to dextral in the upper Keys core. Successive siliciclastic infilling, likely associated with eustatic sea-level change and current redeposition, formed a foundation for subsequent carbonate deposition. Deep-sea biostratigraphic techniques, integrated with ages derived from strontium-isotope chemostratigraphy, can be successfully applied to coastal-margin sequences, even though a depauperate suite of faunal markers is common.

Original languageEnglish (US)
Pages (from-to)653-666
Number of pages14
JournalJournal of Sedimentary Research
Volume69
Issue number3
DOIs
StatePublished - Jan 1 1999

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sand
quartz
strontium isotope
chemostratigraphy
grain size
Pliocene
Miocene
biochronology
carbonate
redeposition
sequence boundary
benthic foraminifera
sea level change
depositional environment
foraminifera
Neogene
benthos
deep sea
chronology
water depth

All Science Journal Classification (ASJC) codes

  • Geology

Cite this

@article{d1e30d2ecb1344668f8b3fb668fdb540,
title = "Chronologic model and transgressive-regressive signatures in the late neocene siliciclastic foundation (long key formation) of the Florida keys",
abstract = "Recent drilling of continuous cores in southernmost Florida has documented a thick unit of upper Neogene siliciclastics subjacent to surficial shallow-water Quaternary carbonates exposed on islands of the Florida Keys. The siliciclastics comprise the Long Key Formation and were identified in two cores collected from the middle and upper Florida Keys. Achronologic model based on new planktic foraminiferal biochronology and strontium-isotope chronology suggests the timing of siliddastic deposition and provides a basis for regional correlation. The chronologic model, supplemented by vertical trends in quartz grain size, pattern of planktic menardiiform coiling direction, and paleoenvironmental interpretations of benthic foraminiferal assemblages, shows that the Long Key Formation contains three intervals (I-III) of varying thickness, grain-size composition, and paleo-water depth. Interval I is uppermost Miocene. The quartz grains in Interval I fine upward from basal very coarse sand to fine and very fine sand. Benthic foraminifera indicate an upward shift from an outershelf to inner-shelf depositional environment. Interval II, deposited during the late early to early late Pliocene, contains reworked upper Miocene siliciclastics and faunas. In the upper Keys, quartz grains in Interval II range from very coarse sand that fines upward to very fine sand and then coarsens to very coarse and medium sand. In situ benthic faunas indicate an upward shift from outer-shelf to inner-shelf deposition. In the middle Keys, Interval II is different, with the quartz grains ranging primarily from medium to very fine sand. In situ benthic taxa indicate deposition on an inner shelf. In both the middle and upper Keys, the upper Pliocene siliciclastics of Interval III contain quartz grains ranging from very coarse to very fine sands that were deposited on an inner shelf. A sequence boundary between Interval I and Interval II is suggested by: an abrupt shift in the strontium-isotope chemostratigraphy; coarsening in quartz grain size above the boundary; an abrupt landward shift in depositional faci{\`e}s in the upper Keys core; and a distinct variation in the predominant coiling direction of the menardiiform planktic foraminifera, from fluctuating dextral-sinistral to dextral in the upper Keys core. Successive siliciclastic infilling, likely associated with eustatic sea-level change and current redeposition, formed a foundation for subsequent carbonate deposition. Deep-sea biostratigraphic techniques, integrated with ages derived from strontium-isotope chemostratigraphy, can be successfully applied to coastal-margin sequences, even though a depauperate suite of faunal markers is common.",
author = "Guertin, {Laura A.} and McNeill, {Donald F.} and Lidz, {Barbara H.} and Cunningham, {Kevin J.}",
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Chronologic model and transgressive-regressive signatures in the late neocene siliciclastic foundation (long key formation) of the Florida keys. / Guertin, Laura A.; McNeill, Donald F.; Lidz, Barbara H.; Cunningham, Kevin J.

In: Journal of Sedimentary Research, Vol. 69, No. 3, 01.01.1999, p. 653-666.

Research output: Contribution to journalArticle

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T1 - Chronologic model and transgressive-regressive signatures in the late neocene siliciclastic foundation (long key formation) of the Florida keys

AU - Guertin, Laura A.

AU - McNeill, Donald F.

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AU - Cunningham, Kevin J.

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AB - Recent drilling of continuous cores in southernmost Florida has documented a thick unit of upper Neogene siliciclastics subjacent to surficial shallow-water Quaternary carbonates exposed on islands of the Florida Keys. The siliciclastics comprise the Long Key Formation and were identified in two cores collected from the middle and upper Florida Keys. Achronologic model based on new planktic foraminiferal biochronology and strontium-isotope chronology suggests the timing of siliddastic deposition and provides a basis for regional correlation. The chronologic model, supplemented by vertical trends in quartz grain size, pattern of planktic menardiiform coiling direction, and paleoenvironmental interpretations of benthic foraminiferal assemblages, shows that the Long Key Formation contains three intervals (I-III) of varying thickness, grain-size composition, and paleo-water depth. Interval I is uppermost Miocene. The quartz grains in Interval I fine upward from basal very coarse sand to fine and very fine sand. Benthic foraminifera indicate an upward shift from an outershelf to inner-shelf depositional environment. Interval II, deposited during the late early to early late Pliocene, contains reworked upper Miocene siliciclastics and faunas. In the upper Keys, quartz grains in Interval II range from very coarse sand that fines upward to very fine sand and then coarsens to very coarse and medium sand. In situ benthic faunas indicate an upward shift from outer-shelf to inner-shelf deposition. In the middle Keys, Interval II is different, with the quartz grains ranging primarily from medium to very fine sand. In situ benthic taxa indicate deposition on an inner shelf. In both the middle and upper Keys, the upper Pliocene siliciclastics of Interval III contain quartz grains ranging from very coarse to very fine sands that were deposited on an inner shelf. A sequence boundary between Interval I and Interval II is suggested by: an abrupt shift in the strontium-isotope chemostratigraphy; coarsening in quartz grain size above the boundary; an abrupt landward shift in depositional faciès in the upper Keys core; and a distinct variation in the predominant coiling direction of the menardiiform planktic foraminifera, from fluctuating dextral-sinistral to dextral in the upper Keys core. Successive siliciclastic infilling, likely associated with eustatic sea-level change and current redeposition, formed a foundation for subsequent carbonate deposition. Deep-sea biostratigraphic techniques, integrated with ages derived from strontium-isotope chemostratigraphy, can be successfully applied to coastal-margin sequences, even though a depauperate suite of faunal markers is common.

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