Characteristics, distribution and diagenetic stages of chert in the La Silla Formation (Lower Ordovician), Argentine Precordillera

Abstract

The late Cambrian -late Tremadocian La Silla Formation is a carbonate unit of the eastern Precordillera in Argentina whose facies indicate a shallow platform environment. Until this moment, there were no studies that referred to the diagenetic evolution of these rocks. The present study involves the characteristics and distribution of the silicification that affects this unit and determines its different diagenetic stages through petrographic (with cathodoluminescence support) and stratigraphic analyses. An early diagenetic chert, in a stage previous to the compactation, was observed. This chert is related to silica-rich seawater in contact with permeable and porous sediments. A later pulse of chert, associated with fracturing, also occurs. The knowledge provided by the characteristics and distribution of chert in these carbonates is significant, especially when considering that the La Silla Formation in San Juan province is the most quarried unit for the elaboration of lime.

Origin of chert within the Turonian carbonates of Abu Roash Formation, Abu Roash area, Egypt: Field, petrographic, and geochemical perspectives

“…The fine crystal size, lack of normal marine fossils, and the presence of evaporites (i.e., gypsum; Figure 6c,d) indicate that these dolomites originated by the dolomitization of lime mud in supratidal-intertidal settings during the early stages of diagenesis (e.g., Beigi et al, 2017;Wanas, 2008). in the microquartz matrix, which corroborates that the chertification process resulted in the replacement of the lime mud matrix by microquartz at a rate much faster than that of dolomite (Figure 12a; e.g., Lawrence, 1993;Raviolo et al, 2009;Whittle & Alsharhan, 1994); and (d) corrosion of dolomites by quartz, which indicates that these dolomites originated prior to chertification (Figure 10h; e.g., Genedi, 1998). The dolomitization before chertification led to increase porosity, which is a good media for silica cement.…”

“…The chertification postdates the dolomitization process as evidenced by (a) enclosure of the chert nodules and bands within the dolomitic limestone; (b) existence of dolomite rhombs at the contact areas between limestone and chert (Figure d; e.g., Giménez‐Montsant et al, ); (c) presence of dolomite rhombs disseminating in the microquartz matrix, which corroborates that the chertification process resulted in the replacement of the lime mud matrix by microquartz at a rate much faster than that of dolomite (Figure a; e.g., Lawrence, ; Raviolo et al, ; Whittle & Alsharhan, ); and (d) corrosion of dolomites by quartz, which indicates that these dolomites originated prior to chertification (Figure h; e.g., Genedi, ). The dolomitization before chertification led to increase porosity, which is a good media for silica cement.…”

“…This can be demonstrated by (a) the alternation of chert bands and carbonate beds is at casual intervals (does not coincide with the stratum boundaries; e.g., Shaaban, ), (b) the inconstant thickness and lateral nonpersistence of the chert bands throughout the single bed (e.g., Genedi, ), (c) the sharp contact between the chert and the host carbonates (Figure a; e.g., Abd El‐Hameed et al, ; Bustillo & Ruiz‐Ortiz, ; Conglio, ; El‐Mansey & El‐Baz, ), (d) the presence of chert within limestones of diverse depositional textures (wackestone, packstone, grainstone, etc.) implies that the distribution of chert is not related to the depositional environments of their host limestones and accordingly attributed the origin of chert to postdepositional changes (e.g., Gao & Land, ; Knauth, ; Maliva & Siever, ), (e) the presence of allochems similar to those identified from the host carbonates but with conspicuous signs of silicification (e.g., Madsen & Stemmerik, ; Maliva, ), (f) the freezing of most of the depositional textures of the carbonate host rocks (Figure g; e.g., Hesse, ; Maliva & Siever, ; Meyers, ), (g) the partial replacement of the lime mud matrix and allochems of the host limestone by silica (Figures f, c; e.g., Bustillo, Delgado, Rey, & Ruiz‐Ortiz, ; Mansour, ; McBride et al, ), (h) the existence of silicified fossils within the host limestones in some horizons (Figure h; e.g., Bustillo & Ruiz‐Ortiz, ; Maliva & Siever, ; Schubert, Kidder, & Erwin, ), (i) presence of opal‐CT in the form of microquartz‐replaced lepispheres (Figure e; e.g., Genedi, ; Maliva & Siever, ), and (j) replacement of dolomite by quartz (Figure h; e.g., Migaszewski et al, ; Raviolo, Barbosa, & Neumann, ; Whittle & Alsharhan, ). Moreover, the presence of different colours of the studied chert (grey, brown, and black) provides insights about the progressive stages of silicification (e.g., Kwiatkowski, ).…”

“…The lack of compaction features in the studied chert demonstrates that the chertification process was most likely performed during an early diagenetic stage (i.e., prior to significant burial and compaction). This can be indicated by (a) the complete absence of stylolites in the Abu Roash chert (e.g., Lawrence, ), (b) the presence of ghosts of uncompacted and undeformed skeletal allochems in the chert (Figure b; e.g., Maliva, ; Raviolo et al, ), (c) the existence of undeformed silicified burrows (e.g., Conglio, ; Figure d), (d) bending of the host carbonate surrounding the chert nodules (Figure b; e.g., Pope, ), and (e) the presence of limestone bedding enveloping chert in the Abu Roash area, denoting that this chert was already formed and consolidated before the compaction of the limestone host rock (e.g., Maliva & Siever, ).…”

“…This can be indicated by (a) the complete absence of stylolites in the Abu Roash chert (e.g., Lawrence, 1993), (b) the presence of ghosts of uncompacted and undeformed skeletal allochems in the chert (Figure 12b; e.g., Maliva, 2001;Raviolo et al, 2009), (c) the existence of undeformed silicified burrows (e.g., Conglio, 1987; Figure 5d), (d) bending of the host carbonate surrounding the chert nodules ( Figure 9b; e.g., Pope, 2004), and (e) the presence of limestone bedding enveloping chert in the Abu Roash area, denoting that this chert was already formed and consolidated before the compaction of the limestone host rock (e.g., Maliva & Siever, 1989b). Hesse (1989) determined the timing of the silicification with respect to the main diagenetic processes of the host carbonates (e.g., dolomitization, cementation, and recrystallization).…”