Browsing by Author "Khanna, Pankaj"

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    Coralgal reef morphology records punctuated sea-level rise during the last deglaciation
    (Springer Nature, 2017) Khanna, Pankaj; Droxler, André W.; Nittrouer, Jeffrey A.; Tunnell, John W. Jr.; Shirley, Thomas C.
    Coralgal reefs preserve the signatures of sea-level fluctuations over Earth’s history, in particular since the Last Glacial Maximum 20,000 years ago, and are used in this study to indicate that punctuated sea-level rise events are more common than previously observed during the last deglaciation. Recognizing the nature of past sea-level rises (i.e., gradual or stepwise) during deglaciation is critical for informing models that predict future vertical behavior of global oceans. Here we present high-resolution bathymetric and seismic sonar data sets of 10 morphologically similar drowned reefs that grew during the last deglaciation and spread 120 km apart along the south Texas shelf edge. Herein, six commonly observed terrace levels are interpreted to be generated by several punctuated sea-level rise events forcing the reefs to shrink and backstep through time. These systematic and common terraces are interpreted to record punctuated sea-level rise events over timescales of decades to centuries during the last deglaciation, previously recognized only during the late Holocene.
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    Transgressive reef morphology evolution: A qualitative and quantitative comparison of Uppermost Pleistocene and Upper Cambrian reefs in offshore and Central Texas (USA)
    (2017-12-01) Khanna, Pankaj; Droxler, André W
    Understanding the morphological evolution of modern coralgal reefs helps decipher the forcing mechanisms that influence the evolution of ancient carbonate reefs. In this study, a comparison between Uppermost Pleistocene drowned coralgal reefs stretching along the south Texas shelf edge and Upper Cambrian microbial reefs in Mason (Central Texas) is developed. Similarities and differences between the two settings, separated by 500 million years, make it an interesting comparison. Among the major similarities, both settings represent low-latitude, shallow water within the photic zone, shelf edge, mixed carbonate siliciclastic systems, whereas the nature of the two reef builders is the main difference. Forcing mechanisms, among them sea level fluctuations, are well established during last transgression since Last Glacial Maximum and directly tied to the morphological evolution of the Uppermost Pleistocene coralgal reefs. The initial transgression triggered a clear switch from shallow coastal siliciclastics to neritic coralgal reefs on the south Texas shelf edge banks. A similar switch is also observed at the base of the Upper Cambrian microbial reefs where the system changes from mixed carbonate-clastic to pure carbonate system. Based on these comparisons, it is demonstrated that an Upper Cambrian transgression led to the establishment of microbial reefs providing it enough accommodation to grow vertically.