Mark Pilkington

Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico

We suggest that a buried 180-km-diameter circular structure on the Yucatan Peninsula, Mexico, is an impact crater. Its size and shape are revealed by magnetic and gravity-field anomalies, as well as by oil wells drilled inside and near the structure. The stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with, and overlain by, breccias that contain evidence of shock metamorphism. The andesitic rocks have chemical and isotopic compositions similar to those of tektites found in Cretaceous/Tertiary (K/T) ejecta. A 90-m-thick K/T boundary breccia, also containing evidence of shock metamorphism, is present 50 km outside the craters edge. This breccia probably represents the craters ejecta blanket. The age of the crater is not precisely known, but a K/T boundary age is indicated. Because the crater is in a thick carbonate sequence, shock-produced CO2 from the impact may have caused a severe greenhouse warming.

Magnetic interpretation using the 3-D analytic signal

A new method for magnetic interpretation has been developed based on the generalization of the analytic signal concept to three dimensions. The absolute value of the analytic signal is defined as the square root of the squared sum of the vertical and the two horizontal derivatives of the magnetic field. This signal exhibits maxima over magnetization contrasts, independent of the ambient magnetic field and source magnetization directions. Locations of these maxima thus determine the outlines of magnetic sources. Under the assumption that the anomalies are caused by vertical contacts, the analytic signal is used to estimate depth using a simple amplitude half-width rule. Two examples are shown of the application of the method. In the first example, the analytic signal highlights a circular feature beneath Lake Huron that has been identified as a possible impact crater. The second example illustrates the continuation of terranes across the Cabot Strait between Cape Breton and Newfoundland in eastern Canada.

Size and morphology of the Chicxulub impact crater

The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.

3-D magnetic imaging using conjugate gradients

A 3-D inversion approach is outlined that determines a distribution of susceptibility that produces a given magnetic anomaly. The subsurface model consists of a 3-D array of rectangular blocks, each with a constant susceptibility. The inversion incorporates a model norm that allows smoothing and depth-weighting of the solution. Since the number of parameters can be many thousands, even for small problems, the linear system of equations is inverted using a preconditioned conjugate gradient approach. This reduces memory requirements and avoids large matrix multiplications. The method is used to determine the 3-D susceptibility distribution responsible for the Temagami magnetic anomaly in southern Ontario, Canada.

Identifying remanent magnetization effects in magnetic data

Remanent magnetization can have a significant influence on the shape of magnetic anomalies in areas that are generally characterized by induced magnetization. Since modeling of magnetic anomalies is nonunique, additional constraints on the direction of magnetization are useful. A method is proposed here to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity. From the amplitude and relative position of maxima in these derived quantities, we infer the deviation of the magnetization direction from that of the ambient field. The approach is applied to the magnetic anomaly in the center of the Manicouagan impact structure (Canada). Our results, based only on the magnetic anomaly observations, are in close agreement with constraints on the direction of remanent magnetization from rock samples.

Gravity and magnetic field modeling and structure of the Chicxulub Crater, Mexico

The Chicxulub crater is an ∼180-km-diameter peak-ring crater based on drill hole logs and samples, potential fields, seismic reflection profiles, and surface fracture patterns. A structural cross section produced based on these constraints has the features expected for a large complex impact crater. The Bouguer-gravity anomaly consists of a broad ∼90-km radius, ∼30-mGal low with a central ∼20-km radius, ∼20-mGal high and two <5-mGal concentric lows at ∼35- and ∼60-km radius. The gravity anomaly is disrupted by large-scale basement anomalies and possibly by large-scale slumping and backwash erosion effects. The magnetic field anomaly over the crater consists of three zones, an outer zone from ∼45- to ∼90-km radius of low-amplitude, short-wavelength anomalies with an irregular perimeter, a middle zone from ∼20- to ∼45-km radius of high-amplitude, short-wavelength anomalies slightly elongated NNW-SSE, and an inner ∼20-km-radius single large-amplitude anomaly. Magnetic field modeling indicates that the melt pool averages ∼90 km in diameter and the melt volume in the crater is estimated at ∼20,000 km3. The melt pool size constrains the collapsed transient cavity diameter to ∼90 km. Gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but ∼40 km in diameter and underlies or protrudes into the melt pool. The preliminary structural cross section indicates that the inferred peak-ring is decoupled from the structural uplift. The geometry and Bouguer gravity signature of the crater indicate that no significant uplift of the Moho or relaxation of the crater has occurred.

Determination of crustal interface topography from potential fields

We present a method for determination of the topography of an interface separating two uniformly but differently magnetized media. The technique is developed within the framework of discrete linear inverse theory and exploits the quasi‐linearity of the problem to derive a simple, efficient algorithm suitable for use on large gridded data sets. Variation of auxiliary parameters allows a suite of acceptable models to be produced rapidly; these parameters can be appraised in light of available geologic and geophysical evidence. Because very large matrices are involved when large amounts of data are interpreted, approximate resolution measures requiring no matrix inversion provide useful information concerning averages of the constructed topography.

Mapping Chicxulub crater structure with gravity and seismic reflection data

Abstract Aside from its significance in establishing the impact-mass extinction paradigm, the Chicxulub crater will probably come to exemplify the structure of large complex craters. Much of Chicxulub’s structure may be ‘mapped’ by tying its gravity expression to seismic-reflection profiles revealing an ∼180 km diameter for the now-buried crater. The distribution of karst topography aids in outlining the peripheral crater structure as also revealed by the horizontal gradient of the gravity anomaly. The fracturing inferred to control groundwater flow is apparently related to subsidence of the crater fill. Modelling the crater’s gravity expression based on a schematic structural model reveals that the crater fill is also responsible for the majority of the negative anomaly. The crater’s melt sheet and central structural uplift are the other significant contributors to its gravity expression. The Chicxulub impact released ∼1.2 × 1031 ergs based on the observed collapsed disruption cavity of ∼86 km diameter reconstructed to an apparent disruption cavity (Dad) of ∼94 km diameter (equivalent to the excavation cavity) and an apparent transient cavity (Dat) of ∼80 km diameter. This impact energy, together with the observed ∼2 × 1011 g global Ir fluence in the Cretaceous-Tertiary (K-T) fireball layer indicates that the impactor was a comet estimated as massing ∼1.8 × 1018 g of ∼16.5 km diameter assuming a 0.6 gcm−3 density. Dust-induced darkness and cold, wind, giant waves, thermal pulses from the impact fireball and re-entering ejecta, acid rain, ozone-layer depletion, cooling from stratospheric aerosols, H2O greenhouse, CO2 greenhouse, poisons and mutagens, and oscillatory climate have been proposed as deleterious environmental effects of the Chicxulub impact with durations ranging from a few minutes to a million years. This succession of effects defines a temperature curve that is characteristic of large impacts. Although some patterns may be recognized in the K-T extinctions, and the survivorship rules changed across the boundary, relating specific environmental effects to species’ extinctions is not yet possible. Geochemical records across the boundary support the occurrence a prompt thermal pulse, acid rain and a ∼5000 year-long greenhouse. The period of extinctions seems to extend into the earliest Tertiary.

3D magnetic data-space inversion with sparseness constraints

I have developed an inversion approach that determines a 3D susceptibility distribution that produces a given magnetic anomaly. The subsurface model consists of a 3D, equally spaced array of dipoles. The inversion incorporates a model norm that enforces sparseness and depth weighting of the solution. Sparseness is imposed by using the Cauchy norm on model parameters. The inverse problem is posed in the data space, leading to a linear system of equations with dimensions based on the number of data, N . This contrasts with the standard least-squares solution, derived through operations within the M -dimensional model space ( M being the number of model parameters). Hence, the data-space method combined with a conjugate gradient algorithm leads to computational efficiency by dealing with an N×N system versus an M×M one, where N≪M . Tests on synthetic data show that sparse inversion produces a much more focused solution compared with a standard model-space, least-squares inversion. The inversion of aeromagnet...

Scaling nature of crustal susceptibilities

Scale invariant or self-similar properties have been demonstrated for a wide range of geophysical processes and rock properties. Evidence from well logs and inferences from aeromagnetic field power spectra suggest that crustal susceptibility also shows this behaviour with a power spectrum proportional to the −4 power of the spatial frequency. We examine two large susceptibility measurement data sets and find that they both support the scaling hypothesis.