PEMBENTUKAN UNDAK BATUGAMPING DAN HUBUNGANNYA DENGAN STRUKTUR DIAPIR DI PERAIRAN TANJUNG AWAR-AWAR PACIRAN JAWA TIMUR

Batuan yang mendominasi di daerah penyelidikan adalah batugamping koral Formasi Paciran zona Rembang. Batugamping koral inilah membentuk undak pantai di Tanjung Awar-awar dan di pantai Tuban dan sekitarnya. Terdapatnya undak batugamping ini, menandakan adanya pengangkatan secara vertikal pada satuan batuan ini. Batimetri daerah penyelidikan secara umum merupakan dataran dimana bagian yang dangkal terdapat di bagian baratdaya (ke arah pantai) dengan kedalaman 2 meter dan terdalam ke arah timurlaut dengan kedalaman 9 meter. Sebanyak 7 lintasan seismik berarah timurlaut-baratdaya dan 15 lintasan berarah barat laut – tenggara telah dilakukan dan beberapa titik bor untuk memperkuat penafsiran jenis batuan di setiap lapisan penampang seismik. Struktur geologi yang ditafsirkan dari seismik pantul ini adalah adanya suatu blok batuan yang seperti tersesarkan dan terdorong ke atas sebagai struktur diapir. Struktur diapir ini berkembang sangat baik di penampang seismik ke arah barat daya atau ke arah daratan dimana di sekitar pantainya batugamping ini membentuk undak batugamping. Munculnya struktur diapir ini kemungkinannya dikarenakan bagian selatan dari zona Rembang ini terdapat suatu zona yang mempunyai anomali gaya berat negatif dan karena batugampingnya banyak terpatahkan sehingga sangat mudah diintrusi oleh sedimen yang mempunyai berat jenis kecil. Kata kunci : undak pantai, batugamping koral, struktur diapir, Tanjung Awar-Awar Jawa Timur The study area is dominated by coral reef limestone of Paciran Formation of Rembang Zone. This coral reef limestone is responsible to form the beach terraces along the Tanjung Awar-Awar and Tuban beach and its surrounded. The formation of this coral reef limestone terraces closely related to vertical movement of these rocks units. The study area is bathymetrically flat where the shallow part is on southwest (towards the beach) with 2 meters depth and the deeper part is to northeast part with 9 meters depth. There are 7 seismic reflection lines of NE-SW and 15 lines of NW-SE have been done and some rocks drilling to emphasize the seismic sequences. The interpretation of geological structure from seismic reflection shows a feature of rocks unit was faulted and intruded as diapiric structures. These features are well developed towards the beach where the terrace of coral reef limestone can be found on the beaches. The formation of these diapiric structures are interpreted where on southward of the Rembang Zone there is a gravity negative anomalies and also due to the limestone were faulted and it seems to be easy intruded by a sediment with low density. Keywords : beach terraces, coral reef limestone, diapiric structure, Tanjung Awar- Awar East Java

Fluid inclusion microthermometry and rare earth element distribution in the celestites of the Jebel Doghra ore deposit (Dome Zone, northern Tunisia): towards a new genetic model

The celestite ore of Jebel Doghra occurs as stratabound deposits within the cap-rock of a diapiric structure of Triassic salt-rocks. The celestite deposits result mainly from the late diagenetic to epigenetic replacement of the carbonated host-rocks giving rise to a dolomite-celestite “banded ore”. Celestite is locally observed within fractures. This study proposes a new genetic model based on fluid inclusion (FI) microthermometry and REE geochemistry. FI show that celestite, occurring either as stratabound bodies or lodes, was deposited from a highly saline (20.7 ± 1.3 wt%NaCl equivalents) and warm (174 ± 3oC) basinal fluid, which contains hydrocarbon droplets and CO2. The geochemistry of the REE shows that the deposition of celestite is due to the mixing between a deep-sourced fluid which has acquired high Sr concentrations by leaching feldspar-rich series in depth and a sulfate-rich solution associated with the Triassic evaporites.

Introduction

The thrilling biblical saga of Sodom and Gomorrah leaves a deep impression on the spirit of its readers, especially the young. Basic ethical concepts such as right and wrong were dramatically portrayed by that simple and cruel, yet humane, story. Memories of even more ancient disastrous geological events apparently were interwoven into the saga. A geologist cannot remain indifferent when investigating the Dead Sea region and observing stratigraphical and structural evidence of past and continuing similar events. Forceful dynamics indicated by vertically tilted beds of rocksalt layers that have penetrated upward through the ground and by later processes that have shaped some beds into pillars trigger association with the ancient story. Such features are abundant and clearly recognizable along the foot of the diapiric structure of Mount Sedom (Arabic Jebel Usdum). A gas blowout during the drilling of a water well near the Amazyahu fault in 1957 only by good luck failed to produce a gush of fire and smoke. Such an event could have happened in ancient times as a natural result of faulting. Knowledge of the regional geological background permits translation of the biblical descriptions into scientific terms, which suggests that the sagas of Sodom, Gomorrah, and Jericho described real events that occurred during ancient times before much was known about geology. Thirty-five years of the authors' professional experience in the Dead Sea region encompasses many geological aspects of the basin: deep and shallow stratigraphy, structural history, seismology, sedimentological processes, and the physical and chemical properties of the water. Archaeological studies in the region are reviewed. Although most of these studies are applicable to exploration for oil and gas or extraction of salts from brines, their results illuminate the role of changing paleogeography and paleolimnology on human environments. Climate changes and lake-level fluctuations have occurred since Mid-Pleistocene, especially during the past 50,000 years. Studies of sediments from shallow core holes delimit coastal areas that when exposed by drops in the level of the Dead Sea, quickly developed soils that could be used for agriculture.