Paleomagnetic and Rock Magnetic Stratigraphy of the Mid- Cretaceous

通过对意大利翁布里亚地区Albian Contessa剖面的古地磁研究，确定了白垩纪正极性超时线中高温赤铁矿携带的几个反特征剩余磁化（ChRM）间隔。相反的间隔对应于整个翁布里亚发现的2个红色带（色素赤铁矿），这可能反映了沉积期间海底的氧化状态。相反的ChRM可能代表在33R（或更晚）反极性沉积后18 m获得的再磁化。如果是再磁化，这个过程是非常有趣的，因为它可以产生类似极性间隔的再磁化模式。另外，反转ChRM可能记录了在中albian （107-104 Ma）强烈的海底氧化事件期间获得的反转地磁场极性间隔。如果正确 , 几个间隔足够的时间(100可以) 使用磁性地层研究和可能提供的标记 在白垩纪的海洋地壳安静的区域。如果是初级的，这些间隔也表明地磁反转频率模式的修正。最近，在Albian Breggia剖面（瑞士）也发现了2个带红色的含赤铁矿沉积物。红色带可能不仅仅具有区域意义；它们可能标志着古海洋学条件的快速变化，类似于白垩纪全球缺氧事件（但在氧化方面相反）。古地磁数据表明，布雷吉亚沉积物中存在反向的ChRMs，由此我们可以排除再磁化情景，即循环流体仅影响部分测量剖面。然而，对先前存在的矿物相的再活化是目前正在考虑的另一种再磁化机制。然而，在分隔良好的地点，在地层狭窄的层段中发现了反向的ChRMs，这进一步支持了原始磁化模型。我们建议将我们的研究扩展到意大利和深海（ODP）的参考剖面，旨在通过岩石磁和旋回地层（层理和地球化学）测量来表征中albian沉积物的古气候特征。首先，我们希望确定氧化层的性质、持续时间和程度。其次，我们希望利用这些数据作为一种手段，严格检查在阿尔比安（和其他）远洋沉积物中观察到的磁化残余成分的锁定机制。

Paleomagnetic study of the Albian Contessa section (Umbria, Italy) has defined several intervals of reversed characteristic remanent magnetizations (ChRM) carried by high unblocking- temperature hematite within the Cretaceous Normal Polarity Superchron. The reversed intervals correspond with 2 reddish zones (pigmentary hematite) found throughout Umbria which may reflect the oxidation state of the seafloor during deposition. The reversed ChRM may represent a remagnetization acquired 18 m.y. after deposition in reversed polarity chron 33R (or later). If a remagnetization, the process is of great interest since it can produce remagnetization patterns which resemble polarity intervals. Alternatively, the reversed ChRM may record intervals of reversed geomagnetic field polarity acquired during intense seafloor oxidation episodes during the mid-Albian (107-104 Ma). If correct, several of the intervals are of sufficient duration (100 kyr) to be of use in magnetostratigraphic studies and might provide markers on oceanic crust of the Cretaceous Quiet Zone. If primary, the intervals also suggest a modification of models of geomagnetic reversal frequency. Recently, 2 zones of reddish, hematite-bearing sediments were also identified in the Albian Breggia section (Switzerland). The reddish zones may have more than a regional significance; they may mark rapid changes in paleoceanographic conditions analogous to (but opposite in oxidation) to the Cretaceous global anoxic events. Paleomagnetic data suggest the presence of reversed ChRMs from the Breggia sediments from which we can exclude a remagnetization scenario calling upon circulating fluids affecting only a portion of the measured sections. Reactivation of a preexisting mineral phase, however, is another remagnetization mechanism which is presently being considered. Nevertheless, the funding of reversed ChRMs in a stratigraphically narrow interval at well-separated sites gives further support to a primary magnetization model. We propose an extension of our studies to a reference section in Italy and the deep sea (ODP) aimed at characterizing the paleoclimate signature of the mid-Albian sediments through rock- magnetic and cyclostratigraphic (bedding and geochemical) measurements. First, we hope to determine the nature, duration and extent of the oxidized intervals. Second, we hope to use these data as a means to examine critically the lock-in mechanism of the remanent components of magnetization observed in the Albian (and other) pelagic sediments.