Revisiting Beachrock Cementation Processes

This project aims to analyze the nature of beachrock cement producing fluids, processes affecting cementation, and geometry of beachrock. Three sites were chosen for analysis along the northwestern shoreline of Puerto Rico, near the cities of Aguada, Isabela, and Hatillo. To address these topics, we used field observations, petrography (conventional microscopy and Scanning Electron Microscopy (SEM), stable isotopes, geochemistry, and geophysical methods. Based on field observations, petrography, and geochemical evidence we conclude that cements found in all studied sites precipitate from seawater in marine vadose (landward) to marine phreatic (seaward) systems. Recrystallization of cements, at depth, was observed in one locality (Hatillo). The 18O of cements at the surface indicate that seawater has undergone evaporation although cements at depth precipitated from normal marine water. Based on the 18O of groundwater and rainfall on northern Puerto Rico we can rule out significant (e.g. detectable) influx of freshwater in all three sites. The 13C values of cements at the surface can be attributed to seawater with dissolved CO2 13C values in the range of 0 to 2 ‰of. However, at depth we infer that oxidation of organic matter has modified seawater 13C to produce lighter 13C values. In Hatillo, the oxidation of organic matter is interpreted as the drive for recrystallization of cements and their significantly lighter 13C values. Presence of abundant microorganisms and fungi or their activity was observed throughout, both through conventional microscopy and SEM. At surface to a few centimeters below, abundant macro flora and fauna are present. Although impact of biogenic activity on cementation cannot be quantified, their abundance suggests that they could have impacted the system by increasing supersaturation through CO2 removal. Microorganisms and organic matter, in many places, provide surfaces for precipitation of cements, and ultimately oxidize and drive d13C depletion. Ground penetrating radar (GPR) proved useful in identifying sand stratal patterns and the sand-beachrock interface. However, beachrock features below the sand-beachrock interface were not imaged due to high radar signal attenuation. GPR survey lines showed no evident patterns between cements and beachrock geometries across the locations. Geometries appear to be influenced more by beach gradient, coastline shape, and depth of ocean than cement traits.

Name thesis:degree_name

M.S.

Discipline thesis:degree_discipline

Geology

Grantor dc:publisher

University of Kansas

Year dc:date.issued

2017

Author dc:creator

• Villaneda-van Vloten, Isabel

Advisor dc:contributor.advisor

• Gonzalez, Luis A.

• beachrock
• carbonate
• cement
• GPR
• Puerto Rico
• stable itotope

Language dc:language.iso

en

Dc Identifier Other

https://www.proquest.com/LegacyDocView/DISSNUM/10255210

OAI identifier oai:identifier

oai:kuscholarworks.ku.edu:1808/37324