Milankovitch Cycles in Upper Jurassic and Lower Cretaceous Radiolarites of the Equatorial Pacific: Spectral Analysis and Sedimentation Rate Curves

Abstract Logs in the petroleum boreholes indirectly records the sedimentary cycles in the deep burial formation. In order to extract and understand the periodicity and cyclicity, it is necessary to process the data by digital signal analysis method. Taking the gamma ray (GR) log as the primary material, an identification approach of Milankovitch cycles in boreholes is proposed in this paper, which is based on the Maximum Entropy Spectral Analysis (MESA). The first stage chooses the appropriate windows for calculating the frequency spectral properties in a short section of the data. In each depth window, the second stage generates the two-dimension frequency spectrum utilizing the MESA. At each depth point, the third stage finds the potential Milankovitch cycles in the one-dimension frequency spectrum, in which the average amplitude spectrum peak would be matched to the ratio of Milankovitch period. According to the frequency and wavelength of the maximum amplitude in Milankovitch cycles, the fourth stage estimates the sedimentation rate controlled by cyclical factor. Finally, the Milankovitch cycles in Lower Member of Miocene Zhujiang Formation in north slope of Baiyun Sag, Pearl River Mouth Basin, are identified and the cyclical sedimentation rate is estimated. The results demonstrate that the proposed method is feasible and effective to identify Milankovitch cycles in boreholes, which may contribute to the other geological researches.

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PALEOENVIRONMENTAL ANALYSIS OF A LOWER CRETACEOUS PALEOLAKE FROM THE IGUATU BASIN BASED ON ITS OSTRACOD FAUNA AND XRF DATA

Palaios ◽

10.2110/palo.2021.006 ◽

2021 ◽

Vol 36 (12) ◽

pp. 356-376

Author(s):

MARCOS ANTONIO BATISTA DOS SANTOS FILHO ◽

GERSON FAUTH ◽

BENJAMIN SAMES ◽

ERIK WOLFGRING ◽

JORGE VILLEGAS-MARTÍN

Keyword(s):

Cluster Analysis ◽

Spectral Analysis ◽

Organic Carbon ◽

Sedimentation Rate ◽

Lower Cretaceous ◽

Middle Part ◽

Sedimentation Rates ◽

Total Sulfur ◽

X Ray ◽

Paleoenvironmental Analysis

ABSTRACT This paper presents the results of a paleoenvironmental study of two Hauterivian–Aptian adjacent sections (Transnordestina A/B) of the Iguatu Basin using ostracods and aided by X-ray fluorescence (XRF), total organic carbon (TOC), total sulfur (S), and spectral analyses. Cluster analysis divided the 10 genera found into two main groups: one composed of Alicenula-Pattersoncypris?-Brasacypris-Hastacypris-Ilyocypris?, and the second composed of Cypridea-Looneyellopsis-Rhinocypris?. The first group is interpreted as representing paleoenvironments with permanent waterbodies, such as lakes, and the second as being indicative of ephemeral settings, such as temporary pools in an inundation plain. XRF analysis using the positive peaks of Ca/Ti and Ca/ΣTi, Fe, Al ratios show a few dry periods, particularly in the lower and middle part of Transnordestina A, between 0 to 175 m and 385 to 475 m, which also display a small number of ostracods. Total S shows several high peaks which might be indicative of gypsum deposition during droughts. Spectral analysis of molar Ti/Al ratio shows two intervals with different sedimentation rates, between 0 and 233 m, and 233 to 836 m. For the first interval, six 100 ka eccentricity cycles were identified; the low number of ostracods for the interval and highly variable Ca/Ti and Ca/ΣTi, Fe, Al values indicate a different sedimentation rate in a drier environment. For the second interval, ten 405 ka cycles were identified; its higher ostracod count and more stable Ca/Ti and Ca/ΣTi, Fe, Al values could be indicative of increasing humidity.

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