Origin and stratigraphic significance of kaolinitic sediments from the Cypresshead Formation: A sedimentological, mineralogical and geochemical investigation.

摘要

Kaolinitic sediments of the Cypresshead Formation (3.4--2.3 Ma), deposited as two distinct shoreface-shelf parasequences in response to sea level falls at 3.3 Ma and 2.5 Ma, define the final stages of a period of siliciclastic deposition which dominated the Florida Platform between 8.6 Ma and 1.8 Ma. Beginning with deposition of the Late Miocene SS2 siliciclastics of Cunningham et al. (2003) and ending with the Late Pliocene cessation of Cypresshead deposition and reworking at approximately 1.8 Ma, episodes of sediment accumulation correlate with two paleoclimatic transitions from; (1) arid conditions during the Late Miocene to continual El Nino conditions during the early Pliocene warm period (∼4.5--3.0 Ma), and (2) continual El Nino conditions to global cooling and the onset of significant Northern Hemisphere Glaciation (NHG) (∼3.0--1.5 Ma). Responding to the interplay of sea-level, sediment supply and accommodation associated with climate instability, the siliciclastics deposited during this Late Miocene through Pliocene interval define a retrogradational parasequence set episodically deposited on the Florida platform over a 6.8 Ma period.;Facies identified within the Cypresshead Formation define the depositional environment as nearshore marine, consistent with a wave-dominated coastline in north-central Florida and a mixed energy coastline in southeastern Georgia. Composed of coarsening-upward sequences, both facies sets define a progradational response to the coastal delivery and subsequent coast-parallel transportation of a substantial siliciclastic flux, which, in turn, correlates with the nonequilibrium landscape conditions predicted by a transitional mid-Pliocene paleoclimate. A potential fluvial-deltaic component of the Cypresshead Formation in southeastern Georgia is proposed based on paleocurrent indicators and the similarities noted for the offshore transition (OST) facies in Georgia and the prodeltaic Miccosukee Formation.;Mineralogically, Cypresshead Formation sediments are highly weathered, but particularly in north-central Florida where local hydrologic conditions coupled with the depositional fabric and microtexture of the sediments have resulted in the formation of at least three kaolinite fractions; (1) in situ kaolinite formed at the expense of feldspars and mica, (2) detrital kaolinite deposited as part of the original clay mineral suite, and (3) near surface recrystallized kaolinite. In situ kaolinite crystallizes via the combined topotactic (transformation) and epitactic (neoformation) weathering of muscovite mica under saturated conditions to produce an enrichment of vermicular kaolinite in Cypresshead sediments. Kaolinite formed in this way exhibits a high degree of order in response to the epitactic nucleation of crystallites on the structurally similar muscovite surface, explaining the correlation between low disorder and small particle-size that has confounded researchers for decades. Additionally, the feldspar dissolution sourcing of the Al and Si necessary for in situ kaolinite crystallization is confirmed by inherited positive Eu anomalies reported for these kaolinites and the presence (and geochemistry) of residual feldspars in basal Cypresshead sediments.;Weathering occurring under oxic, vadose or mixed vadose/saturated conditions results in the degradation of neoformed kaolinite as evidenced by a decrease in kaolinite order and coherent scattering domain (CSD) values consistent with the formation of the near surface recrystallized kaolinite fraction. Forming under conditions of extreme leaching and recystallization, this kaolinite fraction possesses microtextural characteristics suggestive of a pedogenic origin. The trace phases gibbsite, halloysite and crandallite-florencite also form under similar near surface, oxic weathering conditions. The origin of the crandallite-florencite phase is uncertain, but most likely originated from the decomposition of post-depositional pore water organics coupled with detrital mineral dissolution.;Both trace elements and Nd isotopes were analyzed in order to constrain the provenance and broader geochemical characteristics of the Cypresshead Formation and to compare these results with the studies by Dombrowski (1992; 1993) on the provenance of Cretaceous and Tertiary kaolins from the Georgia-South Carolina kaolin district. Of the trace elements used by Dombrowski, only Th and Sc appear to be resistant to significant mobilization and depletion in Cypresshead sediments, with the Th/Sc ratio a robust indicator of provenance composition. Additionally, a significant proportion of Florida Cypresshead clays possess Th/U ratios well below that of average upper crust, and appear to have undergone redox-driven U enrichment dictated by pore water organics. As for Nd results, Cypresshead Formation samples appear to originate from sources intermediate between those associated with Cretaceous "soft" kaolins and Tertiary "hard" kaolins of Georgia and South Carolina. This is consistent with these sediments being a mixture of materials originating from Carolina terrane and Alleghanian granite sources. Additionally, Nd model ages (TDM) for Florida and Georgia Cypresshead samples range between 1.4--0.9 Ga and 1.5--0.8 Ga respectively, with both possessing an average TDM value of 1.1 Ga consistent with the age of Grenville crust.