Late Holocene to Recent aragonite‐cemented transgressive lag deposits in the Abu Dhabi lagoon and intertidal sabkha

Abstract

Modern cemented intervals (beachrock, firmgrounds to hardgrounds and concretionary layers) form in the lagoon and intertidal sabkha of Abu Dhabi. Seafloor lithification actively occurs in open, current‐swept channels in low‐lying areas between ooid shoals, in the intertidal zone of the middle lagoon, some centimetres beneath the inner lagoonal seafloor (i.e. within the sediment column) and at the sediment surface the intertidal sabkha. The concept of ‘concretionary sub‐hardgrounds’, i.e. laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary, is introduced and discussed. Based on calibrated radiocarbon ages, seafloor lithification commenced during the Middle to Late Holocene (ca 9000 cal yr bp), and proceeds to the present‐day. Lithification occurs in the context of the actualistic relative sea‐level rise shifting the coastline landward across the extremely low‐angle carbonate ramp. The cemented intervals are interpreted as parasequence boundaries in the sense of ‘marine flooding surfaces’, but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited. Aragonite, (micritic) calcite and, less commonly, gypsum cements lithify the firmground/hardground intervals. Cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography, cathodoluminescence microscopy and Raman spectroscopy. The morphology of aragonitic cements changes from needle‐shaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar, lath and platy crystals in the inner lagoon. Precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology. Data shown here shed light on ancient, formerly aragonite‐cemented seafloors, now altered to diagenetic calcites, but also document the complexity of highly dynamic near coastal depositional environments.

Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

“…The Abu Dhabi coastal area, located in the southern margin of the Persian Gulf (Figure 1A), represents a low-gradient (<0.1°) carbonate ramp on a modern subtropical epeiric sea (Lokier and Fiorini, 2016). The climate is characterized by high temperature (>50°C in summer), low rainfall (average of 72 mm/yr) and strong seasonal winds (north-northwest Shamals) (Lokier and Fiorini, 2016;Ge et al, 2020a). Within a restricted intertidal environment at Khawr Qantur (Figure 1A), microbial mats are dominated by cyanobacteria, purple sulfur bacteria and sulfate-reducing bacteria (Scherf and Rullkötter, 2009).…”

“…Dominating carbonate FIGURE 2 | Microbial bioherms in Great Salt Lake, Utah, United States (A) Schematic geological map of the Great Salt Lake (modified after Della Porta, 2015 and references therein) showing faults extracted from Colman et al (2002) and distribution of microbial bioherms and ooidal sand from Eardley (1938) and Carozzi (1962). mineralogies are aragonite and high-Mg calcite (Paul and Lokier, 2017;Ge et al, 2020a).…”

“…Toledo, Germany) was used at the Great Salt Lake and Bagni San Filippo. At Abu Dhabi, salinity, alkalinity and elemental concentrations of seawater were also measured following the protocol defined in Ge et al (2020a).…”

“…In the Abu Dhabi coastal area and other places in the southwestern Persian Gulf, typical fibrous aragonite cements are common due to widespread early marine cementation (Shinn, 1969;Paul and Lokier, 2017;Ge et al, 2020a, Ge et al, 2020b. These typical marine cements are characterized by: 1) various sizes and morphologies in intergranular pores; 2) competitive growth, intercrystal porosity, and lack of nanocrystals in composition; 3) dominance in the lagoonal and open marine settings where botryoidal and spherulitic aragonite are very scarce (Shinn, 1969;Paul and Lokier, 2017;Ge et al, 2020a, Ge et al, 2020b. Whereas typical fibrous marine cements show ubiquitous formation in the shallow marine realm, botryoidal and spherulitic aragonites are only abundant in the intertidal microbial mats.…”

Erosion‐initiated stromatolite and thrombolite formation in a present‐day coastal sabkha setting

“…The general stratigraphy in the area of the pool (Fig. 3C) consists of Pleistocene to Early Holocene aeolian mixed siliciclastic–carbonate sands, overlain by a shallow‐marine transgressive carbonate hardground of mid‐Holocene age (Lokier & Steuber, 2009; Paul & Lokier, 2017; Ge et al , 2020), followed by an overlying unconsolidated, to lightly‐cemented, bioclastic rudstone. This sequence is terminated by laminated microbial mats with a polygonal surface architecture.…”