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首页> 外文期刊>Facies >Thick brachiopod shell concentrations from prodelta and siliciclastic ramp in a Tortonian Atlantic-Mediterranean strait (Miocene, Guadix Basin, southern Spain)
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Thick brachiopod shell concentrations from prodelta and siliciclastic ramp in a Tortonian Atlantic-Mediterranean strait (Miocene, Guadix Basin, southern Spain)

机译：托尔特大西洋-地中海海峡（中新世，瓜迪克斯盆地，西班牙南部）的前斜纹岩和硅质碎屑斜坡上的厚腕足动物壳浓度

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Carbonate production by brachiopods in shallow-water habitats is generally expected to be not sufficiently high and temporally persistent to allow them to form very thick and densely packed shell concentrations. The formation of thick brachiopod concentrations requires long-term persistence of populations with high density of individuals, and such circumstances are assumed to be rare especially during the Cenozoic. However, here we show that the large-sized brachiopod Terebratulaterebratula, the most common species in benthic assemblages with epifaunal bivalves and irregular echinoids, formed several decameter- to meter-thick, densely packed concentrations in shallow siliciclastic, high-energy environments, in a seaway connecting the Atlantic Ocean with the Mediterranean Sea during the Latest Tortonian (Late Miocene, Guadix Basin, southern Spain). This brachiopod formed (1) meter-scale, thick, parautochthonous concentrations in a prodelta setting and (2) thin, mainly allochthonous, tide- and storm-reworked concentrations in megaripples and dunes. The abundance of brachiopods at the spatial scale of the Guadix Basin seems to be mainly related to intermediate levels of sedimentation rate and current velocity because abundance and thickness of shell concentrations decline both (1) in onshore direction towards delta foresets with high sedimentation rate generated by debris flows and (2) in offshore direction with increasing levels of tide- and storm-induced substrate instability. Although brachiopods in dune and megaripple deposits are more fragmented, disarticulated, and sorted, and have a higher pedicle/brachial valve ratio than in prodelta deposits, taphonomic damage is still relatively high in prodelta deposits. Terebratula terebratula thus formed thick concentrations in spite of that disintegration processes were relatively intense along the whole depositional gradient. Therefore, population dynamic of this species was probably characterized by production maxima that were comparable to some Cenozoic molluscs in terms of their productivity potential to form thick shell concentrations in shallow subtidal environments. We suggest that temporal changes in brachiopod carbonate production have a significant spatial and phylogenetic component because multiple large-sized species of the family Terebratulidae, which underwent radiation during the Cenozoic, attained high abundances and formed shell concentrations in temperate regions.
机译：通常预期腕足类动物在浅水生境中产生的碳酸盐不够高，并且在时间上不能持久，从而使它们形成非常厚和密密麻麻的壳浓度。浓厚的腕足类动物的形成需要具有高密度个体的种群的长期存在，而且这种情况被认为是罕见的，尤其是在新生代。但是，在这里我们发现，大型腕足动物Terebratulaterebratula是底栖动物群中最常见的种类，具有表生双壳类动物和不规则类棘突类动物，在浅硅质碎屑，高能环境中形成了数米至米厚的密集堆积浓度在最新的Tortonian（中新世晚期，Guadix盆地，西班牙南部）期间，将大西洋与地中海相连的海上航道。该腕足类动物在前三角洲环境中形成了（1）米级，厚实的副浓度，（2）在巨波纹和沙丘中形成了稀薄的，主要是异源的潮汐和风暴改造浓度。瓜地斯盆地空间尺度上的腕足类动物的丰度似乎主要与沉积速率和流速的中间水平有关，因为壳层的丰度和厚度都下降了（1）朝着由沉积物产生的高沉积速率的三角洲前缘的陆上方向。泥石流和（2）沿海上方向，随着潮汐和风暴引起的基底不稳定性增加。尽管沙丘和巨型波纹沉积物中的腕足类动物比零代沉积物更易碎，脱节和分类，并且具有更高的椎弓根/肱动脉瓣比，但三角洲沉积物中的垂体损害仍然相对较高。尽管解体过程在整个沉积梯度上相对强烈，但Terebratula terebratula形成了浓稠的浓度。因此，该物种的种群动态可能以最大产量为特征，就其在浅潮下环境中形成厚壳层的生产力潜力而言，其产量与某些新生代软体动物相当。我们建议腕足类碳酸盐生产中的时间变化具有重要的空间和系统发育成分，因为在新生代经历了辐射的Ter足科的多个大型物种获得了高丰度并在温带地区形成了壳浓度。

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