Black shale deposition and early diagenetic dolomite cementation during Oceanic Anoxic Event 1: The mid-Cretaceous Maracaibo Platform, northwestern South America

Abstract

ABSTRACT. Thin laterally continuous organic-rich dolomitic marlstones were deposited in the extended Late Aptian -Early Albian epicontinental sea of northwestern South America. These intervals are the proximal equivalents of thick hemipelagic black shale-ammonitic floatstone couplets, deposited in the distally stepped, differentially subsiding part of the Maracaibo Platform. The marlstones reflect the dynamic conditions resulting from orbital forcing mechanisms and can be genetically related to (1) minor sea-level changes, (2) proximal turnovers in marine productivity, and (3) sudden climate shifts affecting mid-Cretaceous shoaling upward, shallow marine, carbonate cyclicity. Therefore, the marlstones may well be linked to the multiple environmental perturbations collectively referred to as Oceanic Anoxic Event 1. The interstitial euhedral dolomite has a medium crystallinity, and exhibits unusual textural relations with framboidal pyrite and gypsum. The authigenic mineral assemblage also includes quartz, Ca-F apatite, and barite, which together with the chemical signals of dolomite, point to an unsteady climate regime. Bulk-rock biomarker parameters, rare earth element geochemistry, and iron speciation data point to widespread photic zone anoxia and transient shallow marine euxinia by the time of deposition, with climatic instability driving the delivery of oxidized detritus from the hinterlands. These conditions led to a schizohaline redox stratified environment favorable to dolomite precipitation. In such a depositional setting, the bio-utilization of Fe, Mn, and sulfur for organic matter respiration sustained elevated pore-water alkalinity and pH, and allowed for the pre-compactional growth of interstitial dolomite.

Biogeochemical reappraisal of the freshwater–seawater mixing‐zone diagenetic model

“…The δ 13 C and δ 18 O data from these dolomitic crusts point to the influence of meteoric waters transporting soil‐derived carbon through the flats (Mazzullo et al ., 1987; Teal et al ., 2000). In schizohaline settings, sulphur‐dependent metabolisms were also proposed as key agents capable of buffering the system toward a sustained state marked by elevated pH and alkalinity generation (Petrash et al ., 2016). Notably, in the Pleistocene of the Abu Dhabi sabkha, where brackish groundwater with a meteoric influence admixes with partially evaporated seawater, sulphur‐respiration appears to also be an important driver of cementation by partially ordered, near stoichiometric dolomite in the sediment column of the upper intertidal zone (Chafetz et al ., 1999; Geske et al ., 2015).…”

“…In the case of the latter, subsurface microbes could facilitate dolomitization through: (i) the production of exopolymeric biomacromolecules with strong affinity for Ca and Mg cations and templating properties (Warthmann et al ., 2000; Bontognali et al., 2010; Zhang et al ., 2012b); (ii) efficient generation of alkalinity linked to OM respiration processes and Fe and Mn cycling (e.g. Petrash et al ., 2015, 2016a, 2016b); (iii) release of dissolved sulphide through bacterial sulphate reduction (Zhang et al ., 2012a); and (iv) the availability of transition metal catalysts (Daye et al ., 2019; Vandeginste et al ., 2019). The mixing zone represents an environment where the conditions listed above can locally concur for periods of 10 4 to 10 5 years while the biogeochemically controlled diagenetic front transitions upward and downward through the sediment pile in response to recurrently changing agents, such as climate, hydrology and glacioeustacy (Saller et al ., 1994; Csoma et al ., 2004).…”

“…Other potential trace metal CL modifiers, such as Co, Ni (Machel, 1985) or Ce (Habermann et al ., 1996), are also sensitive to changes in redox states and might contribute to CL zonation. The initial release to porewaters of redox sensitive elements, such as Ce, that can contribute to chemical zonation in dolomite crystals formed under predominantly anoxic, schizohaline conditions has been linked to the biogeochemical reductive dissolution of Fe‐(oxyhydr)oxides and Mn‐(oxyhydr)oxides (Petrash et al ., 2016a,b). High spatial resolution and precision techniques that can systematically advance the understanding of these metals and their role in CL zonation of mixing‐zone dolomites are now readily available (for example, ion and photon probes, notably synchrotron‐based X‐ray absorption and fluorescence).…”

Formation of lacustrine dolomite in the late Miocene marginal lakes of the East Mediterranean (Northern Israel)

“…Similar climatic control on lithology is familiar from other schizohaline environments (i.e. fluctuations between fresh to hypersaline waters), with favourable conditions for dolomite deposition during drier periods (Folk & Siedlecka, ; Mazzullo & Friedman, ; Varol et al ., ; Petrash et al ., ). Using a sedimentation rate of 5 to 7 cm in 10 3 years (Rozenbaum et al ., ), a duration time of ~100 to 200 kyr for the Tortonian wet periods (corresponding to limestone thickness between 5 m and 10 m) and ~10 to 40 kyr for the Tortonian dry periods (corresponding to dolostone thickness between 0·5 m and 2·0 m) was calculated.…”

Clinoform identification and correlation in fine‐grained sediments: A case study using the Triassic Montney Formation

“…For example, a state of low relative sea‐level has been identified in Late Cretaceous deposits that correlates with increased terrigenous input, low kaolinite/chlorite + mica ratios, high Sr/Ca ratios and high TOC; conversely, high relative sea‐level may generally be associated with the inverse of these indicators (Li et al ., ). This correlation of detrital input with relative sea‐level has also been demonstrated in Cretaceous deposits (Adatte et al ., ; Chenot et al ., ; Keller et al ., ; Petrash et al ., ). In fact, changes in clay content and detrital input have been recognized as providing reliable relative sea‐level proxies, as well as climate, even in deep marine environments (Li et al ., ).…”