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Early Dolomitization of Platform Carbonates and the Preservation Of JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B5, PAGES 7977-7986, MAY 10, 1993 Early Dolomitizationof Platform Carbonatesand the Preservation of Magnetic Polarity DONALD F. MCNEILL Division of Marine Geologyand Geophysics,Rosenstiel School of Marine and AtmosphericScience, Universityof Miami, Florida JOSEPH L. KmSCHVn,•K Division of Geologicaland PlanetarySciences, California Institute of Technology,Pasadena Resultsfrom a combinationof techniquesare presentedto evaluatethe natureof magnetizationin shallow- water platform carbonateswhich have undergonerecrystallization during early calcificationand dolomitization. Magnetic grain separates,coercivity spectra,modified Lowtie-Fuller tests, magnetizationefficiency, and magnetostratigraphicconstraints indicate that the ultrafine-grainedmagnetite is preservedduring early burial geochemicalregimes, inversion from aragonite/high-magnesiumcalcite to low-magnesiumcalcite, and even pervasivedolomitization. These single-domaincrystals are thoughtto occuras interactingmultigrain clusters, someof which may exceed1 /am in diameter. These large clustersmay help prohibit magneticreorientation during diagenesis. Furthermore,during both fabric preservingand fabric destructivedolomitization, the ultrafine-scalereplacement process restricts reorientation of the clusters,thus preserving depositional or early postdepositionmagnetic orientation. This early dolomitization(matrix stabilization)may evenhelp protectand extendthe subsurfacelifespan of the originalpolarity. INTRODUCTION shallow-watercarbonates, this reportwill focuson the natureof magneticremanence in rockshaving undergone early, pervasive, Recognitionof depositionalor early postdepositionalremanent near-surfacedolomitization. We will addressthe questionof magnetizationin Tertiary and older shallow-watercarbonates is magneticgrain preservation through recrystallization, and whether becoming increasinglymore critical. These results are now the magneticgrains are susceptibleto reorientationduring the commonlyused for magnetostratigraphicstudies, paleomagnetic recrystallizationprocess. Samplesfor this study, and data on poles, and for timing of remagnetizationrelated to major which comparisons are based, come from a larger diageneticevents such as: dolomitizationby relatively low- magnetostratigraphicstudy where magneticreversals have been temperaturefluids; very early recrystallization;dolomitization by correlatedto the geomagneticpolarity time scale,with the aid of deep basinal fluids related to tectonic/orogenic events; biostratigraphicmarkers [McNeill et al., 1988; McNeill, 1989]. dedolomitization;base metal emplacement;and early hematite The core borings for this study are from several different formationfrom iron hydroxides[Horton et al., 1984; McCabe et Bahamianplatforms (Figure 1), and penetrateinto Plioceneand al., 1984; McCabe et al., 1985; Elmore et al., 1985; Jacksonand late Mioceneage rocksat their base. Core locationsinclude Little Van der Voo, 1985; Bachtadseet al., 1987; Hurley and Van der BahamaBank (GB-2, WC-1, SC-1); Great BahamaBank (Unda, Voo, 1987; Jacksonet al., 1988;]. In general, magneticgrains U-l, U-3); and Ocean Drilling Program site 632. Holocene incorporatedin shallow-watercarbonates are often subjectedto sampleswere collectedfrom GreatBahama Bank at JoultersCay, severalcarbonate diagenetic fluid and recrystallizationregimes Androstidal flat (GBB-3), Tongue-of-the-Ocean(GBB-4), andLee during deposition, initial dewatering, early compaction, StockingIsland. Magnetizationin shallow-waterrocks which have cementation, mineralogic inversion, and recrystallization. undergone very early dolomitization (within 1-2 m.y. Shallow-water limestonesare especially prone to numerous postdeposition)are of especialinterest since (1) theserocks have diageneticenvironments due to their proximityto sealevel, with undergoneseveral (minimum of two) of the common carbonate extremesranging from an entirelymarine fluid burial, to meteoric diageneticenvironments (initial burial, normal marine fluids, fluid exposure,to a complicated,often repetitivemixed fluid freshwatervadose and phreatic, mixed marine and freshwater); (2) burial history. Secondly, the original mineralogiesare often theymay provide a mineralogicallystable matrix for the magnetic completely altered shortly after deposition, with complete remanencecarriers which could help extend the lifespan of the recrystallizationfrom an original aragonite/high-magnesiumcalcite depositional magnetization, shielding it from completedestruction mineralogyto low-magnesiumcalcite, and dolomite. or remagnetization;(3) they comprisea large portion of late As a result of a better understandingof these carbonate Tertiary carbonateplatforms and atolls which contain a rich diageneticsettings [Longman, 1980; Mcllreath and Morrow, archiveof sealevel and regional depositional events; and (4) there 1990], and an expanding application of paleomagnetismto are similar depositsin the ancient record that may contain importantpaleomagnetic poles if an early magnetizationcan be confirmed. Copyright1993 by the AmericanGeophysical Union. LaboratoryMethods Papernumber 93JB00353. All rock-magneticand magnetostratigraphic measurements were 0148-0227/93/93 JB-00353 $05.00 conductedat the CaliforniaInstitute of Technologyusing a 2G 7977 7978 MCNEILL AND KIRSCHVINK: MAGNETIC POLARITY IN EARLY DOLOMITE I I I I I 79ø 78ø 77ø 76ø 75 ø SC-1 LITTLE BAHAMA 27 ø- BANK GB- 26 ø- JOULTER•" •BB-3 25 ø- UNDA ODP SITE ANDROS 632 24 ø- GREAT BAHAMA BANK LEE STOCKING 23 ø- •L L Fig. 1. Locationof core boringscontaining limestone and dolomite, and sedimentsamples used in this study. Core borings includeGB-2, WC-1, SC-1 from Little BahamaBank; cores Unda, U-l, U-3 from Great BahamaBank; and Ocean Drilling Program site 632 in Exuma Sound. Holocene sample localities include JoultersCay ooid shoal, Andros tidal flat (GBB-3), Tongue-of-the-Oceanperiplatform (GBB-4), and Lee StockingIsland platform sediments. Enterprises 760 and SCT superconductingmagnetometers, polarity continuity,and reversaltests) and rock-magnetictests can respectively. Additional measurementfor the magnetization be used to characterizeand assessprimary versus secondary efficiency tests [Fuller et al., 1988] were done using a 2G remanence. In core boringsfrom recentcarbonate platforms and Enterprises 755 magnetometerat the University of Miami. atolls, the rock-magnetictests become significantly more critical Coercivity and anhystereticremanent magnetization (ARM) tests for assessinga primary remanence. were both conductedon bulk samples,about 3 cc in size. The Several recent studies [McNeill et al., 1988; McNeill, 1990; magneticseparates were isolatedfollowing the techniqueof Chang Aissaoui et al., 1990] have suggestedthat biogenicallyformed and Kirschvink [1985], and examined on a Phillips 300 magnetiteis the primary remanencecarrier in isolated,shallow- transmissionelectron microscope at 100 kV. Magnetic samples water carbonate settings. Definitive recognition of biogenic were collected from zones that have not undergoneobvious magnetitein sedimentsolder than a few thousandyears is otten massivedissolution and reprecipitationassociated with meteoric not possible due to destructionof the characteristicorganic diagenesis. These thin, vertically restricted zones of obvious componentsof the bacteria, mainly the magnetosomeand the diagenesis,usually associatedwith subaerialexposure, included break-up of crystal chains which contain progressivelysmaller void filling calcite and secondary iron-oxides associatedwith crystals in the formative stages at the end of the chain. ancientsoil horizons(paleosols). Recognitionis however basedon a combinationof characteristics suchas limited single-domainsize rangeassociated with known bacteria[Kirsc:hvink, 1982; McNeill et al., 1988], a titanium-poor Sourceof Magnetic Minerals and CarbonateMagnetostratigraphy crystalcompø•ition, crystal shape, and orientationin chains (althoughrarely preservedin platform carbonatesexcept for 3 to Confirming the preservation of an original remanent 4 grain chains,and bearingin mind that the separationtechnique magnetizationand polarity data is critical to applying magnetic may realignthe grainsin chains). Carbonaterocks shownto have reversalstratigraphy to platformcarbonates. In outcrop,both beenremagnetized, have authigenicmagnetite grains significantly conventionalfield tests(such as fold tests, brecciatests, lateral different to those from modern sedimentsand nonremagnetized MCNEILL AND KIRSCHVINK:MAGNETIC POLARITYIN EARLY DOLOMITE 7979 limestone/dolomites.Magnetic grains in these carbonatesoccur the hostphase minerals (aragonite, high-magnesium calcite, low- as either: large (1-100 #m) spheres[Wisniowiecki et al., 1983; magnesiumcalcite) drives the replacementprocess. The pressure Horton et al., 1984; Bachtadseet al., 1987; E#nore et al., 1987; the growingcrystal exerts against the hostphase is responsiblefor McCabe et al., 1987; Hart and Fuller, 1988]; pyrite/magnetite the hostphase dissolution at the stressedface througha solution spheres[Suk et al., 1990]; noninteractingsingle-domain grains of film (several nanometersthick) at the boundary. At the stressed perhapsspheroidal morphologies [Jackson, 1990]; or asextremely contacthost phase dissolution occurs through increased solubility fine-grained(< 300 •) magnetitenear the superparamagnetic-resulting from increasedGibbs free energy. This mechanismis singledomain boundary[Jackson et al., 1992]. intriguing for fine fabric preservationin
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