Depositional environment and diagenesis processes of the amber-bearing rocks in El Valle area (EMD). Stratigraphy, palaeontology and sedimentology of amber-bearing deposit and rocks of the Yanigua Formation have been studying for many years25,28-32,38,63. Based on these results, following depositional systems of the Yanigua complex have been proposed: (1) lagoon to coastal32,37; (2) shallow water marine to marshy lagoon (with presence of calcarenites containing large number of fauna species, oxidised fragments of plants, and transient conditions from anaeorobic to aerobic25; (3) transition from lagoon to carbonate shelf sedimentation (the Yanigua Formation turns to the Los Haitites Formation)25; and (4) shallow-marine environment separated from the open sea, locally marshy environment and floodplains31.
In the most studies, despite the differences in proposed depositional systems, it was concluded that amber was deposited in a low energy31,38 and low salinity water environment63.
Further constraints on the depositional system of the Yanigua Formation may be revealed on the basis of mineralogical data related to composition and microtextures of ore mineral assemblages. Pyrite is often proposed as a proxy indicator of precipitation environment64-67. In this studies, different generations of pyrite were identified based on their specific morphology: framboids, sunflowers, euhedral and detrital grains (Figure 6A-D, F). The most symptomatic for palaeoenvironmental interpretations is framboidal pyrite because it forms in a multi-step, redox-dependent process66 that is linked to syngenetic68 or early diagenetic stage of host rock alteration69. Framboidal pyrite is formed in course of consecutive reactions in presence of different oxygen concentrations66. Thus, the oxic/anoxic boundary provide the most favourable environment for crystallization of framboidal pyrite form. More detailed data on the redox conditions of framboids precipitation pathways might be revealed from distribution pattern of their diameters70. In case of sediments from EMD, the large size of framboids (up to 100 µm) and the presence of another forms of pyrite, rather suggest diagenetic origin of this mineral, which was probably formed under oxic or dysoxic water column71. However, if framboidal pyrite was formed during pseudomorphic replacement of certain organic compounds, the determination of depositional system may be ambiguous. Thus, alternating anoxic/dysoxic conditions cannot be entirely excluded as evidenced by geochemical fingerprint of associated organic matter, especially low Pr/Ph ratio72,73 (Table 3). The oxygen depletion may occur periodically as a result of upwelling process, which played an important role during the formation of the YF complex30.
The dominant presence of pyrogenic quartz with minor contribution of plagioclases and K-feldspars (orthoclase, sanidine) as well as abundant occurrence of titanium oxides may indicate the igneous rocks of Los Ranchos Formation (diorites, quartz porphyries)74 as a potential source area for clastic material of studied rocks. The exposure of this complex is located about 1 km far from the SC-02 borehole, so the clastic material was then transported over short distance. The Los Ranchos Formation also consists of volcanoclastic rocks (tuffs) that might provide the chemical constituents necessary for formation of clay minerals such as Ca-Mg smectite. Additionally, monazite [(Ce, La, Nd, Pr, Gd)PO4] was determined in studied rocks. This mineral usually occurs in acid, oversaturated igneous rocks and their tuffs, as well as metamorphic rocks such as schist and gneiss. It is very resistant to weathering processes, therefore it is also a common component of sedimentary rocks. Some monazite crystals are regularly washed by the sea waters and accumulate in deltaic, beach or shallow marine sediments75-79.
The absence of illite in investigated rocks suggest a very low degree of diagenesis80,81. The presence of mirabilite in the rocks also indicates a shallow diagenesis because this fragile mineral is unstable under higher temperatures82.
Only small amounts of aromatic compounds were found in the samples, no naphthalene and its derivatives are present, whereas phenanthrene and its derivatives are in low concentrations. Therefore, most of maturation indices cannot be calculated.
However, maturation of organic matter may be indirectly estimated from the formula proposed by Jarvie et al.83,
![](data:image/png;base64,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)
where:
Cal %VRo – calculated vitrinite reflectance equivalent,
Tmax – maximum temperature measured due to the Rock-Eval.
A cal %VRo value of 0.38 (averaged for samples SC1 and SC2) is indicative for very low degree of OM maturation.
Comparison of Dominican amber-bearing regions. The two mining districts found in north (NMD) and east (EMD) of the Dominican Republic seem to be various sedimentary basins of one Pre-Ocean, separated from each other by a network of tectonic dislocations formed as a result of intense tectonic movements during the Early to Middle Miocene25. Over the time the epeirogenic processes triggered the mass wasting, causing the sedimentation of terrigenous (clastic-organic) sediments in nearby basins.
For the rocks of the El Valle area (EMD), the presence of Ca-Mg smectite, carbonates (calcite, aragonite, dolomite) or sulfates (mirabilite) indicates a saltwater environment of deposition of terrigenous material. The reservoir was likely subjected to denudational movements, which promoted cyclic sedimentation processes of plant detritus and resins. This is also supported by the rich fossil contents, mainly dominated by marine fauna remnants, such as mollusks, ostracods, foraminifera, bryozoans, red algae, echinoids, and so on. However, the clastic material of rocks probably originated from the Los Ranchos complex, found in close vicinity of amber-bearing sediments in the El Valle region.
In the case of La Cumbre deposit (NMD), the presence of kaolinite indicates an oxic environment of low pH, resulting from progressive accumulation of organic matter. Framboidal pyrite started to precipitate when the concentration of oxygen was lowered to anoxic conditions. The results of facial and mineralogical analyses of amber-bearing strata suggest that the environment of marine sedimentation was likely transitional between a shallow maritime lake and periodically flooded plain10. The clastic material probably came from the elevated rocks of the Pedro Garcia complex25-28,84.
The significant accumulations of plant detritus, derived from the same botanical source, i.e. the Hymenaea protera (Fabaceae) tree species, are found in the rocks from both mining districts. In the sediments from the La Cumbre (NMD) the plant detritus consists of larger tree fragments, branches and fruits. This material has undergone strong alterations, firstly rotting in a highly oxidised environment, and then the transformation at more reducing conditions. As a result strongly altered sediment with locally occuring characteristic soft lignite was formed. Contrary to the rocks from the El Valle (EMD), the nature of the plant detritus is quite different. It is mainly small fragments of plants, leaves and even grasses, well preserved and showing internal structure which is diagnostic for wood. It might be the result of long-term rest in seawater, which has well-known good preservative properties.
Along with the plant detritus provided from the land to the sedimentation basin, amber crumbs originating from tapping trees were also accumulated. The present study and works of others10,57,85-87 have shown that microhardness and density values of resins from the Hato Mayor Province (EMD) are slightly higher than those from the Santiago Province (NMD). In addition, besides typical amber, exceptional resins fluorescing blue and green in white light are found in both mining districts.
Analysis of biomarker provides further information on the conditions of organic matter deposition as well as its genetic source of the rocks from both regions. The primary indexes used in the reconstruction of palaeoenvironment were: n-alkanes distribution, CPI, Pr/Ph, TARHC, Pr/n-C17, Pr/n-C18, C27-C28-C29 steranes and Paq. ratios.
The Carbon Preference Index (CPI) is commonly used for the determination of the source of n-alkane and maturity of organic matter88. Immature source rocks with significant input of land-plant organic matter are usually dominated by the odd-carbon-numbered n-alkanes, particularly n-C27, n-C29, and n-C31. These n-alkanes originate from epicuticular waxes and they are either are synthesized directly from higher plants or defunctionalized even-numbered acids, alcohols or esters89. In the Siete Cañadas area, the samples from SC-02 borehole have CPI values ranging from 1.26 to 2.67 with the sample SC3 showing the highest value (Table 3). These results suggest that the source rocks had mixed terrestrial/marine organic matter sedimented in anoxic and dysoxic depositional environment59,90-92. comparable to n-alkane CPIs for the estuary sediments93. For the reference, the rocks from the La Cumbre deposit exhibit a slightly higher CPI values (Table 3) that suggest the presence of the mixed origin of organic matter, in large portion of terrestrial material. The environment conditions during deposition were generally more oxic than in Siete Cañadas area.
The properties of the isoprenoids, natural hydrocarbons mainly of plant origin, have been also used in identification of the deposition environment and source of organic matter. The higher concentration of pristane than phytane was observed in the rocks from the Siete Cañadas area. The calculated pristane/phytane (Pr/Ph) ratio was in the range from 0.49 to 0.71 (Table 3). In the organic matter from La Cumbre deposit, the concentration pristine and phytane was almost the same, the calculated ratio was in range from 0.92 to 1.01 (Table 3). In the case of rocks from the Siete Cañadas area, the very low values of the Pr/Ph below 0.8 (Table 3) suggest anoxic/hypersaline or carbonate environments72. The values of Pr/Ph in the range of 1.0-2.0 suggest a dysoxic environment 89-91. The calculated Pr/Ph ratio for rocks from La Cumbre deposit may suggest that organic matter was deposited under transitional, between anoxic and oxic conditions. Hence, it can be concluded that the organic matter was accumulated in both areas mostly under reducing conditions, in the presence of low oxygen concentration.
The terrigenous/aquatic ratio (TARHC)62, and Paq. Ratio94, also were used as an indicator of terrigenous and aquatic organic matter components. In samples from Siete Cañadas area, TARHC ranges from 1.34 (SC1 sample) to 8.56 (SC4 sample) with median of 4.25. TARHC values 6.05 and 7.04 were observed for samples from La Cumbre deposit. Despite the differences in values of TARHC, the terrigenous component, from debris of higher plant, is clearly visible in both areas. The Paq. values fall in the range from 0.19 to 0.38 for rocks from Siete Cañadas area. The lower values, i.e. 0.13 and 0.15, were found for rocks from the La Cumbre deposit (Table 3). Ficken et al.94 reported that the Paq. values ranging from 0.01 to 0.23 are diagnostic for terrestrial plant waxes, whereas the values in the range 0.48-0.94 for submerged/floating macrophytes. The results obtained for Siete Cañadas area suggest the dominance of submerged/floating macrophytes, whereas
Paq values for rocks from the La Cumbre deposit indicate the higher plant/macrophyte waxes.
C27, C28 and C29 steranes from both regions have similar distributions (C29>C27>C28) (Table 4), and indicate a terrestrial source of organic matter (Figure 11). Only in the samples SC1 and SC2 from the Siete Cañadas, higher amounts of pentacyclic terpanes were found which suggests presence of plankton/algal organic matter fraction.