墨西哥湾东北部海区常压与超压沉积盆地 孔隙水的地球化学特征对比研究*

Brazos-Trinity IV盆地和Ursa盆地均位于墨西哥湾东北部海域,两者相隔300km,经历了截然不同的更新世沉积历史。Brazos-Trinity IV盆地属于更新世晚期浊流沉积体系,沉积速率低,几乎不产生超压,而Ursa盆地受密西西比河流域的影响,具有极高的沉积速率,是一个典型的超高压沉积盆地。国际大洋综合钻探计划IODP308航次分别在两个盆地取得了钻孔样品,分析获得了沉积物孔隙水的各项地球化学数据。文章以这些分析结果为基础,讨论超压Ursa盆地U1322钻孔和常压Brazos-Trinity IV盆地U1319钻孔沉积物孔隙水中阴阳离子随深度的变化情况,并对孔隙水中保守性组分进行了相关性分析。对比研究发现U1322钻孔的碱度,Cl^-,PO^3-₄和pH值明显低于U1319钻孔; 碱度与PO^3-₄,Ca²⁺,B³⁺之间以及K⁺与Li⁺在U1319钻孔呈现很好的相关性,而在U1322钻孔则无明显的相关关系; U1322钻孔孔隙水Cl^-随深度逐渐降低,孔隙水被淡化。超压Ursa盆地沉积物孔隙水阴离子异常的主要原因可能是因为盆地底层的Blue Unit砂体将高和低超压区连接,流体在超压作用下由高压区流向低压区,阴离子含量较低的流体与沉积物孔隙水混合,造成U1322钻孔中阴离子浓度的异常,这可能也是U1322钻孔孔隙水保守性组分之间相关性较差的主要原因。

COMPARISON OF GEOCHEMICAL CHARACTERISTICS IN SEDIMENT INTERSTITIAL WATER FROM NORMAL-PRESSURED AND OVER-PRESSURED BASINS IN THE NORTHEAST GULF OF MEXICO

The Brazos-Trinity IV Basin and the Ursa Basin are located in the northeast of the Gulf of Mexico, approximately 300km apart. The two basins have different Pleistocene sedimentation history. The Brazos-Trinity IV Basin belongs to a latest Pleistocene sediment gravity flow depositional system without over-pressure due to low sedimentation rate. Its fluid properties in this normal-pressured location would serve as a baseline against the properties measured in an over-pressured basin. The Ursa Basin is located within the Mississippi Canyon. It has high sedimentation rate and is a typical over-pressured basin. IODP Expedition 308 drilled cores in both basins. In this study, we discussed chemical variations of the interstitial waters for both the site U1319 in the Brazos-Trinity IV Basin and the site U1322 in the Ursa Basin. We found out that the alkalinity, Cl-, PO3-4, and pH in interstitial waters in the site U1322 are lower than those in the site U1319. In the site U1319, there exist good relationships between alkalinity and PO3-4, Ca2 , B3 contents, but such relationships are generally absent in the site U1322. The geochemical anomalies and poor relationships between conservative ions in sediment interstitial waters in the Ursa Basin are suggested to be related with basin fluid flow, which may occur laterally within the Blue Unit. The Blue Unit is the base of the deepest sheet sand and is the hydrologic communication of high and low pressure. The gradual interstitial water freshening in the site U1322 may indicate mixing between in-situ interstitial waters and fluids from the Blue Unit at this site. The flux of water is from high pressure to low pressure, and the fluids with low anions' concentrations from the Blue Unit may have mixed with the sediment interstitial water, which is the main reason for the geochemical anomalies in this over-pressured basin.